YAY! I figured it out, thanks to you! It seems like Ctrl+F10 and Shift+F10 were bound to some WoW actions. That was the problem. One of my addons has this as a default hotkey, and I think I installed it after I started with my multiboxing. Very stupid mistake, but at least I have learned from it. A lot!

Thanks for your help, very much appreciated. Now all I have to do is to wait for my copy of Win7 64bit and my RAM modules to arrive and I can start with my five-headed hydra.