So it turns out Widnows 7, and I guess probably Vista, have this silly system with Windows Updates where it keeps backups of all of the outdated updates on your hard drive. Eg you install update A, then update B comes along that replaces A, Windows automatically backs up A just in case you ever uninstall B and it can roll back to it.

Since uninstalling updates is probably only done by 0.01% of people ever, this constitutes a lot of wasted HD space. A few months ago Microsoft finally put out a new update that lets you delete all of these update backups safely.

Here's the digs: http://howto.cnet.com/8301-11310_39-57608286-285/delete-windows-update-files-to-regain-hard-drive-space/

This is mainly of interest to those with SSDs for their system drives; I've only got 64gb, but I managed to free 4.55gb of these update backups, a pretty decent saving.

This should be common knowledge. (But I guess it's not, so good on you for posting it.)
The tool has been around for over 10 years although I can't say when it had which features along the different OSes, SPs and updates. But I do think it was possible to clean up these update backups in each of the Windows versions (XP does the same) in some way.

So if the guide doesn't apply to your windows, try google. If that doesn't help I might be able to assist because I'm sure I've done it in XP and Windows 7 although I can't recite how without looking into it. (If you have Vista don't bother. Format c:\ works much better in terms of cleaning up a hdd. )

I think Windows 8 has had this, but I don't recall ever seeing that option in disk cleanup on anything earlier. I'll have to check it out on my Windows 7 on an 8 GB USB flash drive to see if it shows up now.

CCleaner also deletes these. Glad I didn't wait for Microsoft.

I was trying to delete these files about a year ago when my SSD was getting low on space. The only advice I could find was that if you deleted the directory they were stored in , it would likely break windows because that directory has a mixture of currently-in-use things as well as all this backup crap.