Whereas I like the interface of the Windows 7 Paint, I find that there is one annoying feature when compared to the XP version. If you wish to modify an image, and you magnify the necessary section(s) and turn the grid lines on, you will find that the pixels and gridlines line up in the XP version, making editing relatively easy. Unfortunately, in the Windows 7 version, they do not. I wish they would.
That said, you should find either Paint.net or The GiMP suitable choices.