That's the method I use, using PrtScn to make 
screencaps...muchcheaper than buyin a program ^_^ 
However, there is a much easier way to capture 
the active window, rather than having to cut it 
out of you whole desktop ^.~ To capture just the 
active window, you pres "Alt" and "PrtScnSysRq" 
at the same time, and viola! You have the active 
window. Hope this helps!

-Misha-chan