I could do that in a split second in Photoshop, just by making a selection
with the Marquee tool and Ctrl+Alt dragging it over to the right side, but
I have absolutely no idea if Paint can even do that.
If you do not want to splash out on Paint Shop Pro or the cheap version of
Photoshop (Photoshop Elements), I think you'd be much better off with
Saint Paint, or similar, which is free. MS Paint is seriously rubbish.
http://www.saintpaint.com/