When you install Adobe Reader 8 on Windows Vista you see the error message, "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."
You have turned off User Account Control or you are logged in using the default administrator account.
Acrobat trial and Adobe Reader use NetOpsystems Nosso optimizer to ensure the download files are not corrupted. This utility extracts the contents to the user temporary folder. Microsoft Windows Vista, with User Account Control disabled, does not allow write and execute access to the %TEMP% folder.