Let us talk about copying contents first:
Since you've mentioned that you are using 'msysgit' on Windows XP
Option 1: Click on the Topleft icon > Defaults > Select "QuickEdit Mode" under "Edit Options" > Ok. Now select the text you want to copy and then press Enter.
Option 2: Click on Topleft icon > Edit > Mark. Press Enter.
Now that we know how to copy, let us talk about pasting the copied content:
Also, if the "QuickEdit Mode" is on, Right clicking might work as well.