There's a free program called Image Resizer. Can be had for XP and Win7 and I presume other O/S'. Once installed all you do is right click on the image, and you will now see a new selection called, resize image. They have four preset sizes or you can do custom sizes and it will save the resized image, a copy of the original, to the same folder where the original is.

http://imageresizer.codeplex.com/