pretty weird, I already responded to this, and yet now I can't find what I had entered before ...
oh well, let's try this again ...
MS Paint (in WinXP) can handle JPG files. But by default, it doesn't appear so. Once you open a JPG file with MS Paint, you will be able to save in not only as JPG but also GIF, TIF, and PNG.
It is just an oddity of MS' Paint.
Now, once you get MS Paint to understand it can work with JPG files, then I suggest that you get IrfanView, a really GREAT graphics (over 60 formats reading, and over a dozen formats writing) viewer and converter.
(it also can make stand-alone slide shows, and is a very nice screen capture tool, it will display various video and sound file formats as well!)
Anyway, once you have IrfanView loaded you can set it up to easily (keypress Shift-E) to open any existing graphic with a defined external editor.
So you pre-define that to be MS Paint, and there you go.
A nice graphics viewer and converter, and any time you need to do a quick edit, that IrfanView can't do for you, just "Shift-E" to open it in MS Paint.
Cool, huh? 