I have Vista Business on my laptop. I download JPG pictures from my Nikkon camera. I open them with mspaint and resize them. Then save them back as jpg (using the jpg option). When I try to open the saved file, I get "Paint cannot read this file. This is not a valid bitmap file". When I preview it with windows photo gallery, I can see the picture. Any ideas?
How are you saving them as .jpg ? Did you select JPEG in the file format options? (I believe paint defaults to .bmp, so you may have a .bmp save with a .jpg extension).
However, as people have already said in the comments, Paint is not a great program for any serious image editing tasks. A good free alternative is Paint.NET, it has most of Paint's ease of use, with a much larger feature set.