Making a screenshot:
Hit [Prt Scr] key, open MS Paint, hit [Ctrl] + [V].
Edit pic: In tool stack click upper right button (Finnish term "select"), select area you want to save as a screenshot, hit [Ctrl]+[C], open new sheet [Ctrl]+[N] and paste selected area by hitting [Ctrl]+[V].
If sheet's white area is too large there's small black dots in the corners and you can resize the sheet by dragging from those.
Save as a jpg or a gif and you're done.
If you're having a "real" proggie like Photoshop or Paint Shop Pro you can paste the pic to them instead to MS Paint.
Another way: Download
Hypersnap , that proggie can take screenshots even from 3D-accelerated games and has plenty of options how to edit the pic and in what format to save it.