What exact version of avast did you try to install?

On one of the machines, I even did a Vista repair install, which then meant I had to re-install all the Windows Updates.
If you run a small support company, you should know that with a slipstreamed cd/dvd all updates would have been installed during the repair install.

And Windows Vista? I take it they are old(er) systems. Put at least XP on them and not the most bugged OS MS has ever released.

And please read THIS