Can't install anything on my Windows 7, can't write to temp

I don't know what happened, but all of a sudden I can't install anything on my Windows 7 computer. I have admin privileges, and that annoying Windows permission prompt thingy is turned off. Tried turning off Norton Antivirus. Had no problems before installing anything. A sample of messages I get: * Cannot get/create writeable temporary file folder * Error writing temporary file. Make sure your temp folder is valid. I went to my Local/temp folder, and installation files did show up in there, so it appears writeable, but ??? Winning bid will describe how they'll troubleshoot this, with a best guess as to what the problem is - simply for me to gauge your understanding. After winning the bid, please send me a few instructions to try. If those don't work, I will need you to remote into my computer to fix the problem with me. Let me know if you have a remote support tool.