Installing another hard drive with a different OS

I have a second computer that seems to install World of Warcraft perfectly (the only problem is that it doesn't meet the requirements to run it) but my main one doesn't. So I was thinking that if i just take the hard drive and put it into my main computer, that I'd be able to play. It has windows 2000 installed and my main has XP. Is it as simple as hooking the hard drive up and then just selecting which OS to use when I start my computer up?

No. Whenever you change motherboards, you 99.999999999999999% of the time have to reload the OS. Windows just doesn't take the change well.
What you could do is wipe the hard drive, install the game on that and use the second hard-drive as just that, a second hard drive. I doubt that will get you anywhere thou...

But the problem is that I can't install it on my main computer. How exactly would I do this??