U need to reinstall windows (or mac) to fix this. Get out the cds that came with the pc, and u should find one to reinstall windows. If you can't find the disk, email your pc manufacturer and they will send u one. Make shure that when you reinstall, you format the hard drive instead of using the current partition.
here is a guide: http://windows.microsoft.com/en-US/windows/help/install...
get a program called tree size and see what files are taking up the most space that you don't need and delete them and then empty the recycle bin also.
here is info on treesize
TreeSize Professional is a powerful and flexible hard disk space manager for Windows. TreeSize Professional will help you to find out which folders (and files) are the largest dissipaters on your drives and to recover megabytes of disk space.