How can I play on a WoW private server using Windows Vista?

Well, I used to play on a WoW Private servers on my Windows Xp. But now I got a new computer and it's Windows Vista, but I can't find a way to play on the private server.
I was able to find a way to change the realm list, but now evertime I try to log in it says:

"Unable to validate game version..."

Does anyone know what to do?

Private servers are against Blizzards TOS. If you have an active account with WoW, you agree to this every time you logon after a patch. Your account can be banned for playing on a private server. You might want to weigh whether losing access to your characters is worth it.