Cannot install Parallels Tools

I upgraded to Parallels 5, but when trying to upgrade the Parallels Tools, after the Tools upgrade downloaded and tried to install I was getting a Windows error message: Installer could not be opened. Contact the vendor....
Anyway, after a couple hours of trial and error I freed up some disk space (disk space was getting low) and that did the trick. Tools installed OK and appear to be working fine.
Just in case someone else has this problem, this is how I resolved it.