The only way I personally know of to really restore a hard drive to its original state (providing it's not actually damaged) is to reformat and restore.
However, there are some things you can do to clean it up quite a bit (and do them regularly as maintenance):
-- Delete all temp files (c:\windows\temp\*.tmp)
-- Delete temporary internet files (c:\windows\temporary internet files\*.*)
-- If you use I.E., click on Tools, Internet Options, Delete Files, select "delete all off-line content", click OK
-- Click on Start, Programs, Accessories, Systems Tools, Disk Cleanup
-- Download
AdAware, check for updates, run it and remove whatever it finds
-- Download
Diskeeper and defragment the hard drive
Hedda Lora