What are the names of the temp files that were so large? More applications than just Windows use that folder - I just checked mine (on a PC I've used daily for years and that hasn't been rebooted in a month) and it's about 379 MB. (Win 7)
The files were nonsensical files with HEX names, 20 or 30 characters long. So it looks like they were generated by the OS.
I scanned the system with an AV package, it couldn't find anything suspicious.