If I was to change the motherboard, would it erase everything from my computer? Like all the games I have, would I have to reinstall it?

Also, if I was to get a new computer, what is the best way to transfer all my files (Music, pictures, maps, etc).

You fail quite hard sir. Mobo=/=Harddrive.
Prolly an external HDD for the second.

Pretty much what Anonymous says. The motherboard is basically the control center of your computer, that connects to the inputs, the outputs, and everything else, like processors, that make the computer run. Your datas are stored on your hard drive, and yes, an external HD is probably your best choice. You can use a flash card, transfer to another computer, or store online if you don't want to pay. You might even be able to use an Mp3 player as a hard drive if you want, or the SD memory card in your camera.

It's possible, though, that some drivers might not function with another motherboard (I think it is, at least - I'm not certain), so... yeah. Wait until someone like Excalibur posts to be sure.

Changing a motherboard without changing everything else will screw you over.

In most cases, your current hardware wont fit on it. Or the Mobo is too slow/fast and is wasted.

I've changed one like, 3 weeks ago.

If you're ONLY changing mobo, then no, your files are kept but you need to reinstall everything.

As far as Windows, some motherboard changes will work fine with Windows still working fine afterward. Some do not. If you are changing to a similar board (like a motherboard chipset from the same company), that has the highest chance of still having a functioning Windows after the switch.

Other than similar chipsets, sometimes changes to a different maker's chipset can still work (for example, I once went from nVidia to AMD without issue). With some swaps sometimes a driver for a component on the previous board can crash when using a chipset from a different maker.

In such cases it may be possible to get it working by changing devices to use their generic driver instead of the one specifically for the chipset; I've had this work in my experience, but I also know of others who have only been able to get it partially working and had to disable some devices if they didn't want to reinstall. To do this, it needs to be done while still using the previous board, though. At worst it could need a reinstall, but you can do that without reformatting.