I have had this problem in the past when using a seperate PCI card, currently with a dual head I don't run into it. Here is what I did to fix it, though it is a bit wierd.
The upper left corner of your screen is the (0,0) grid point. If your res is 1024x768, then the lower right corner is (-1024,-768), though in reality I believe the computer calls these positive points. Using the display properties, I setup my second monitor as directly oposite of my main one. IE, it's lower right corner is (0,0). This means that in order to scroll into that monitor, I have to move my mouse directly into the upper left corner of my main screen. This takes some getting used to, but it allows me to play games and not click outside my main screen thus exiting the game.
On a side note, this also prevented my second screen from changing resolution when my main screen did, like when playing a game at a different res than your desktop. That way my temp monitor and trillian windows would stay there and allow me to see them.
I know this is a ghetto solution, but it worked well for me!
-spike