How to change "temp" folder.

[rage4order]

I'm running Vista right now and I'd like to change my "temp" folder root to another drive. Reason is, I'm trying to install Adobe Reader and every time it gets done extracting the files, I can't install because the Windows Installer box pops up and it says something about the Temp folder being on a drive that is full or inaccessible even though I have over 14Gb of space on that drive. It also says to verify I have write permission to the drive, but I am running the program as the administrator.
Any ideas?