I had troubles installing Adobe Reader 8 in Vista and was receiving the following error: "The Temp folder is on a drive that is full or is inaccessible. Free up space on the drive or verify that you have write permission on the Temp folder."
I found the solution...You need to download the install file to your desktop, right-click the file, goto Properties->Compatibility Tab and select 'Run this program in compatibility mode for: Windows XP (Service Pack 2)'
Then press OK and double click the downloaded install file and you should be good to go!
I found this solution here.