Well, it may be something else.....
Here on this forum in the past have been articles and links to windows privacy issues.
Apparently Windows stores a copy of EVERY email message from Outlook and EVERY url you have used in a series of HIDDEN files.
Sorry for shouting, but I didn't want people to say that just by cleaning out your cookies and temp files they are removed.
Some information is here....
http://www.gi-ad.com/winskupa/default.asp
I use History Killer from
www.historykill.com, but it does not erase the big hidden files. It does have an excellent popup stopper though....
I've thought for the past year that some computers simply slow down in spite of constant scandisk, defgrag and registry cleaning because those hidden files approach a size where the Windows Swap File has to be shrunk....thus the computer slows down....