Dual monitors on a HD3870

I have a
HD3870 and I was thinking about buying a second monitor so that I could for example
talk in MSN while playing a game, so connected a old monitor to the second DVI
port to test if it's going to work properly before spend my money on another
monitor. The problem is that when I'm playing a game in full screen, if I move
the mouse to the other monitor and click on a window the first monitor go back
to the desktop. So here is my question, is there a way for me to, for example,
type a message in MSN in the second monitor without having to go back to the
desktop in the first monitor?

Doesn't it depend on the game? Some will and some won't, I think. Some games I have a mouse cursor that can move into any monitor I want. Problem I can not scrool in the game. Others I do a control-alt-delete to get out of the game and back to the desktop. It pauses/minimizes the game - so I have to resume it later.