Find and delete all:
*.tmp
~*.*
and everything found under Temporary Internet Files ... If you have a History file under your profile, then delete it's contents as well. Then run through a scandisk and a defrag.
Aside from general cleaning of files and maintenance on the hdd, are you noticing anything else? Like noises coming from the hdd as you are trying to boot or run Windows Explorer? If there are noises, you might want to take in your computer to a repair center -- your hdd may be going bad.
Slowdowns can occur because of corrupted files on the hdd, too many files (and not enough contiguous free space), or because of hardware problems. If you regularly clean off Temp files, scandisk, defrag, etc... then you may have ahardware issue with the hdd.