First, if you're running XFire (the chat thing from xfire.com, not ATI crossfire), disable the in-game features. If you're running Mumble with in-game overlay you may get a similar performance issue as with XFire in-game.

Then, what Oatboat said.

Then since you gave nothing to go on and I know nothing about how you set up your windows before, I guess I would point you at the Quick Start Guide: http://isboxer.com/wiki/WoW:Quick_St...considerations

If none of that helps, then: What kind of PC do you have? (CPU, GPU, RAM, version of windows) how many monitors?