So for awhile now, I have a pop-up telling me that my Virtual Memory is nearly full(?) or used up(?)
But since these pop-ups, my computer has been slowing down, to the point that SC is lagging, and freezing.
Is there any way that I can fix this? I looked into it online, and it told me to do a disk clean-up.
I did... It was running for a good 38 hours, and the progress bar was only on the third one, like so: [|||______]
So I closed that down, and now I'm wondering if there is anything else I could perhaps do to fix this?

Stuff like that takes time. Let it run for a while.

Backup your documents, maps, etc. and do a reformat/reinstall for your OS. That's really the best and easiest way to get a burdened system unburdened.

First off, please share what OS you are using.

Yeah, if it's taking 38 hours to do disk cleanup, then you might be past the point of no return. In which case a format and reinstall will bring you back and might end up being more efficient in terms of saving time.

(Note, if you're using Windows XP and disk cleanup is getting stuck on a step such as "compress old files", you can do start -> run -> cleanmgr.exe /sageset:1 to set the options for a run of disk cleanup and then start -> run -> cleanmgr.exe /sagerun:1 to run with the options you just set. This might also work for newer versions of Windows, but I am not familiar with them.)

Assuming it's Windows, I would go into the taskbar and close all nonessential programs. Then run Task Manager and close all of the nonessentials that are sucking up your memory. Then I would go completely through all steps in a comprehensive malware removal guide. (If you suspect there is ANY chance your computer has malware, as the guide states, it is not a fix for slow computer.) As for basics of gaining space and speed, uninstalling unused/old programs, regular usage of CCleaner, and defragmenting on a regular basis are all important. More advanced users may want to disable unecessary Windows services, look into replacing memory-hog programs with more lightweight versions (for example, using something like Trillian instead of multiple messaging clients), use more powerful registry scanners and a registry defragmenter, overclocking, spend hours tweaking every setting in Windows for max performance, etc.

How much RAM does your system have? How much does task manager say is being used? Either you don't have enough or you have too many programs running (background or otherwise).

As SF suggested first give us an overview of the situation of used and total RAM. You might just need to upgrade. Newer programs/content usually use more RAM so over time your memory demand will slowly grow.
Also check your free disk space. I suspect the C:\ drive is almost full because that explains both the virtual RAM message and the disk-cleanup / defrag freezing. If it is, delete all files in the recycle bin and go Start -> Run -> and enter "%temp%" (without the quotes) and delete everything there as well. Ignore files that can't be deleted. This often immediately frees up a few GB. Then check C:\ usage again. If you still have less than 10% free disk space you'll have go into more detail when hunting for unnecessary files. After you have enough free space run disk-cleanup and defrag.

If nothing else turns up I'd just try using another disk-cleanup / defrag program.

If that fails as well either go Roy's path if you can afford a fresh system (less work / less need for reading up) or follow Mooses recommendations (no need to backup and reinstating workspace) to revitalize your current operating system.

Let's see... I have Windows XP and I did what Moose said with the disk clean-up. The clean-up worked this time around, finishing in under 7 hours, and it felt as if my computer starting running slightly better again, but now I'm not so sure. RAM is.... I'm not really sure what RAM is or how I check it. So I'll hop into my Harddrive GB total, used and unused. C:\ Capacity: 159,989,886,976 bytes (149 GB) ¯ Used space: 91,712,008,192 byes (85.4 GB) ¯ Free space: 68,277,878,784 bytes (63.5 GB)
After my disk clean-up, I think I cleared out about 5-7 GB. So it's not like my Capacity is filled up or anything, I use Malware Bytes' Anti-Malware, and I've tried running full system scans, but about 12-14 hours into it, the scan crashes. It never finds any virus', but it always crashes. I've run quick scans, and they all come up clean. (And don't crash). My second Anti-Virus nearly always is running, because my temporary files(?) are potential virus'(?) and it nearly is always finding and quarantine(ing?) them.
Right now, I think I'm up to atleast 300 or so quarantined temporary files. So should I go into my temporary files, and just delete all of them?

RAM stands for Random Access Memory. It is not stored permanently and clears whenever the computer is not powered. It is essentially your computer's "working" memory. I'm not a computer science expert, so I can't explain how it works in detail, but maybe we can say that the amount of RAM a computer has means how much data your computer "knows" at a time without having to read from a hard drive or elsewhere. A lot of programs can display your RAM information... it's displayed clearly in the top in CCleaner, is visible in Properties for My Computer (which is a different way of doing start -> run -> sysdm.cpl), in start -> run -> msinfo32.exe, in Task Manager under the Performance tab, where it is called Physical Memory.

The nonsense that's going on already indicates that malware is playing some role and that your computer needs a good malware cleanup as detailed in the guide which I posted. It'll take a good day or maybe even two days (a lot of which will be waiting for scans to finish and programs to run), but it will be worth it. A format will destroy it for good. (unless you have backed up something malware-infested or it's nested inside whatever you back up) Reinstalling XP fresh may be a compromise in terms of time around it, but some malware is advanced enough to survive through a fresh install and continue plaguing your system.

I saw you refer to your "second Anti-Virus". Someone with more knowledge may wish to correct me, but as far as I know there is almost no reason to run two anti-virus programs simultaneously. They can often interfere with each other in addition to sucking up unecessary system resources/memory. Keep the one that's either "better" or that you prefer and get rid of the other(s). (Note, this doesn't mean scanners and whatnot, only full antivirus applications like McAfee, Norton, AVG, Avira, etc.)

Also, seriously, run CCleaner. It'll automate clearing out all of the temporary files you can delete as well as many other things.

And uninstall things you don't use and delete large files that you don't need/use anymore.

And yes, odds are that you can delete the quarantined files that your antivirus is keeping. Unless they have data which you need or you want to analyze them to better understand the malware or virus that they contain.

It's recommended before you do anything except formatting your harddrive that you run a defrag program. Because whatever you do -- especially scanning for files with CCleaner, virus scans etc. -- will take up to 10 times longer on heavily fragmented hard drives.

You can actually change virtual memory directly. It seems strange that this would 'suddenly' start happening, so someone (or thing) may have changed your virtual memory setting to something too small.

Do the following:
1. Right click my computer,
2. Go to properties
3. Click advanced
4. Click settings in the performance section
5. Go to advanced tab
6. Find how much virtual memory is says you have

Once you find out your RAM size, your virtual memory should be set to 2x the size of your RAM, eg if you have 2gb of RAM you should set your virtual memory to 4gb.

IMO, uninstalling unused/useless programs and CCleaner should come before a defrag. There is no point defragmenting a bunch of things that will be deleted anyway, plus the defrag itself will go faster with fewer files and less data on the drive.

These articles might also be helpful:
How to Make Your Computer Run Like New in 4 Easy Steps
How to Optimize That New Computer

Though the TC still needs to do a thorough malware cleanup or format and fresh install.