I take it you want to play a game with the screen resolution of 3840x1080? Not possible with your current setup unless you get yourself a Matrox DualHead2Go (or TripleHead2Go) or and ATI 5000 series card.
I have heard of some software program that allows this function (I forget the exact name unfortunately) but it was a real pain in the back to get to work and compatibility was close to nonexistent. You would be much better off with the Matrox or ATI solutions.
Hmm, i went and grabbed my other monitor just to test this out. It seems they changed things a bit. The settings I was referring to are now located by right clicking on your desktop, and selecting screen resolution. Give that a go.
Unfortunately there doesn't seem to be a desktop stretch mode anymore. Just Extend. I gave Supreme commander a go and it works, but only because it supports multi-monitors.
Which game/s are you trying to do this with?
If that doesn't work, i've heard Ultramon works wonders for multi-monitor setups, i'd give that a try, plus it fully supports win7 last i heard.