Here are a few things to gain a bit of space and speed :-
1. On a regular basis run Disk Cleanup and delete Temporary Internet Files.
2. In C:\Windows\Temp delete any *.tmp files that are more than a day old.
3. If you use AdAware remove all the (except the latest) *.bckp files from from C:\Program Files\Lavasoft\Ad-aware 6\Quarantine and all the (except the latest) *.txt from \Logs.
4. If you use Grisoft AVG remove all the (except the latest) *. Avg files from C:\Program Files\Grisoft\AVG6\Log and all the (except the latest) *.bin files from C:\Program Files\Grisoft\AVG6\Update
5. If you're not using Norton anti-virus remove Taskmon via Msconfig. Also remove the contents of C:\Windows\Applog where a count of the number of times a program is used is stored. It supposedly speeds up the defragmenter but I haven't noticed any difference.