Perhaps you cant do this, im sorry for that...
Press ALT+Pr Scrn button, u will get the Active window. But u cant get part of the screen
Recommend u try PCHand Screen Capture, it could help u capture anything on the screen. Besides, it offers seven capture modes including Rectangle mode.U can set the width and height as ur need. It provides a build-in editor with which u can edit and save the captured images. Im using it, it is easy to use, even for a Novice. I think it's pretty good ...Hope it could help...
http://www.screen-capture-record.com/