bram1253: Anything in the temp folder is safe to delete.
mqstout: IT guy here. While this is a good heuristic, this is not always true.
Sometimes programs/installers are badly done and put meaningful things in the temp folder. Usually wait until a few reboots and 24 hours have passed before deleting things you're not sure about. Sometimes there's a "have to update on next reboot" staged in the temp area that won't actually apply until later, and it can break things if you delete temp before it applies. It's pretty rare nowadays, but still happens.
I know that what I said is not 100% true, but what will break (pretty much nothing) should work again after a reboot.
I sometimes clear my temp folder and nothing has happened so far. If a file is being used then your OS will refuse to delete the file and that's that.
Anyway even though it's theoretically possible for stuff to break, pretty much nothing will break.