because photoshop is a colour aware program but windows itself is not... therefore, if the image is say in a colour space other than sRGB, or if there are profiling issues, then photoshop would show colour different to windows... this of course begs the question, what to do now... first, profile your monitor, then for images to be viewed outside of photoshop, convert the images to sRGB colour space... this should reduce colour differences...
