I am running Vista and I have Virtual PC 2007 with a VM running XP. When I try to capture the screen with Alt+PrintScreen I get nothing. When I use PrintScreen I get a screenshot of the entire screen, but it takes extra time to crop all the images. Is there a way to get the Alt+Printscreen function to work inside a VM that is running Windows XP ?
I am guessing that you are selecting the Window and then your Alt+Print Screen is getting captured in the guest.
You have two options...
Failing this, it may be easier to either to just do print screen as normal and crop it or use a third party screen grabbing software (Just Googled and that one with the link looks ok, but I recommend you do aa bit of research yourself if you want to go down this route).
The correct way is actually given here:
Depending on what version you have.