It has been quite a while since last post. I upgraded to Windows 7, even since beta, and henceforth I couldn't capture any screenshots from DirectX games via the Prt Scr (Print Screen) button. All I used to get was a screenshot of the desktop instead. Tried quite a bunch of DirectX screen capture tools as well, but in vain.
Recently, I had to tweak program settings to get the latest 'Need for Speed - Shift' working on Windows 7 (upgraded to Windows 7 Ultimate 64bit finally). One of the changes I had to do was to change the program settings to "Disable desktop composition" and "Disable display scaling on high DPI settings". I got my NFS working fine and now I can take game screenshots using my print screen button too... voila!!! I found that if you select/mark the "Disable desktop composition" option in the settings of the exe (game launcher) you can take screenshots while the gaming is running. Take print screen, ALT + TAB to come to desktop and paste the clipboard content into some image editor (for eg: IrfanView/Paint).
All the best, hope it helps you as well!