I would guess display problems, try test with dif. refresh rate, resolution - may be even let game start up in XP compatibility mode. Turn Vista effects on/off. Trial and error. Running dxdiag.exe give no errors? You have Rivatuner or other tweaker? Latest Nvidia drivers? May be try a total uninstall/reinstall of Nvidia stuff. Not the worlds most stable drivers. In Window mode there are no problems with alt-tab right? This happens with both opengl and directx games?
Site is down for me right now but this forum http://www.nvnews.net/vbulletin/index.php
is full of Nvidia freaks. Try search it for "alt-tab". Have seen it mentioned before, like years ago when I played games ;) I remember a tool called powerpro which made a certain game crash to desktop - for some strange reason. What you have running might be problem but only suggesting to check. Have no idea.