Link, paint.net is free, and is like an easier version of gimp, lol12w, I made our sig and avatar using paint.net(and some other programs), its extremely simple to make a straight line, pm me if you want help.
If you want a good armband, I would use gimp. Not saying you can't use paint, but it won't give you more than very simple projects and I am not very accustomed to the tools, so no help there. If you start using gimp and want help, you can shoot me a pm aswell. I have taken classes for this sort of thing.