1) Make sure the window you want is active, and hit Alt+PrintScreen (usually above your Insert key, but keyboards are different)

2) Open MSPaint.

3) Hit Ctrl+V.

4) Save as a JPG image.

5) Attach it to your message (use the Full Featured Reply). If it's too large or an image, use MSPaint's Stretch/Skew function to set it at 75% of it's original size. If it's STILL too large, crop out the left part of the window.