How can I stop Microsoft Paint from changing the pixels in my images?

Every time I save a drawing I've done in Microsoft Paint, the next time I open it the pixels around where I've coloured have been changed. For instance, drawing a single blue box on a white background changes the pixels around the image to a lighter blue, and some of those in the box change in colour too. As I've been doing rather small artwork, every pixel counts, and the effect this produces is rather annoying. Is there a setting somewhere I can change so that it does not do this?How can I stop Microsoft Paint from changing the pixels in my images?
When you save, make sure you save the image as a 24-bit bitmap, not 256 colors or JPEG.