Move your My Documents folder to the drive with 100GB. Here is how to do it:
Also look in your Windows folder, you can delete all the $NtUinstallKBXXXXXXX$ (the Xs represent arbitrary numbers) files EXCEPT the $hf_mig$, leave that one. These files refer to Windows updates (these files are used to uninstall any update that messes up your PC) if your PC is running OK you can delete all those files.
My comp is about 5 years old and has 3 drives, the C drive is 17, D drive is 100 and other is 65gig. I've always saved, downloaded and installed eveything on the other 2 drives but my C drive some how only have abot 700Mbs free. I've gone through all the basic steps, suck as remove programs, disc clean up, virus, adware, spyware scan and tried to defrag but since I only have 4% free it doesn't finish. I've even gone through folder by folder looking for things to delete and have no clue whats taking up all the space...my comp runs extremely slow because of the very limited space on the C drive and I despretly need some help PLEASE.........:(