one of the first things that i do when i install a new windows os, is to change the locations where windows stores its temporary files.
control panel >> system and security >> system >> advanced system settings >> environment labels >> change both TMP and TEMP variables.
i change both of these to c:\windows\temp. i then put a shortcut to that folder in my explorer favourites, so it's easy to find.
as part of my regular maintenance routine, after a fresh reboot, i visit the folder, press Ctrl-A to select all files, delete them, and then empty the recycling bin (i also have a shortcut to this in my faves). this is particularly recommended before defragging.
lots of programs and installers dump files here, but forget to delete them. the files are perfectly safe to delete, after rebooting.
hmm, maybe i should make this into a tutorial, i guess not many people will find this here...