Using TreeSize Free, I found that I had over 4 GB of files taken up in:
C:\Documents and Settings\LocalService\Local Settings\Temporary Internet Files\Content.IE5\
All those sub-folders are temporary, but they haven’t gotten cleared out. CCleaner doesn’t touch this folder, either. Very odd. I deleted them and haven’t noticed anything wrong yet. *knock on wood*
Thunderbird users — Also, I had over 500 MB taken up in the INBOX file in my Thunderbird profile folder. (Search for INBOX.msf and you’ll find the associated INBOX file, bloated. It doesn’t have an extension.)
I use IMAP, though!
C:\Documents and Settings\Bryan\Application Data\Thunderbird\Profiles\xxxxxxxx.default\ImapMail\my mail server
I closed Thunderbird, added a prefix (OLD) to each of them (to be safe), then reopened Thunderbird. New INBOX and INBOX.msf files were created — at 40 KB. Yay! I closed Thunderbird once more, and deleted the old INBOX files.
Disclaimer: Don’t hold me responsible if your computer goes up in flames. I’m a tinkerer. Your mileage may vary.
All Narfed Up 
My new Windows XP laptop has 32 KB in the Content.IE5 folder. =)
I guess you were using Internet Explorer often… I doubt you’ll fill this folder up to 4 GB again since you now use Firefox.
Another fix is to write a startup/shutdown script that checks this folder and deletes anything over a certain age.
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@Arnold:
I don’t use IE often. I saw some CRW (Canon) RAW files in some of the folders, too, so it didn’t *entirely* have to do with me using Internet Explorer. Very weird.
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