IrfanView...it's free and works a treat. I use it all the time. When you've got a screen you want to capture, hit PrtSc, then open IrfanView, hit Ctrl+V, and Bob's your uncle.
Use the mouse to capture the bit you actually want, then go up to Edit and scroll down to Crop Selection. Then save in whatever format you want. It supports almost anything.
It's also the program I use to change a photo or graphic from one file format to another.
Great program! Everyone should have it. ;^)