This small program installs in most versions of Windows. You just have to right-click a photo or group of photos. Then, select Resize Pictures.
You will get a screen with some preset image sizes. Click the Advanced button to enter a custom image size.
The program automatically makes a copy of the photo to resize. But you can tell it to resize the original as well. I don't really recommend resizing the original, though.
http://imageresizer.codeplex.com/