Wy dont u run something like CCleaner, get rid of the trash and then run chkdsk and defragment. System restore points can take up space. Delete ur temp files, recycle bin files, etc and if u hve stuff u can burn them to CDs and make more room. If you add and delete files a lot ur drive must be badly fragged, which is why the slowdown.u better defrag regularly lese once the disk space gets full, u will hve fewer troubleshooting options left. Also as everyone said,spyware SCANS! also check out these links
http://www.microsoft.com/AtWork/getstarted/speed.mspx/
http://www3.telus.net/dandemar/slowcom.htm