What type of mouse is it?
So you've installed all the parts, the computer is up and running, and Windows is installed?
You could try BIOS updates maybe, although only try it if you think it will actually help.
Is it a wired or wireless mouse? If it's wireless try an old wired mouse and see how far that gets you. Or check the batteries if it's wireless.
Does you mouse come with a software cd? Sometimes they have the driver on it.
I also though that windows comes with a generic driver that it uses until you get a more specific one
You can always redownload+install drivers if there arent there.
It's some crap mouse but i know it works, b/c it does on my laptop
Well i had to replace the motherboard and so when i put the new one in and plugged in all the parts the mouse wasnt working, so i couldnt download the drivers,so then i got a USB stick and downloaded all the drivers from the interent and still didnt work.
I already have my BIOS up to date
Its also a wired mouse, and it doesnt work with my other mouse either.....