MS Paint

This utility is part of windows and is usually found under Programs/Accessories/Paint from the start bar. The results are not the best from this as the graphics engine is pretty basic. You may get some distortion but if you have nothing else on hand it will do the job well enough.

Open the image you want to resize, go to the Image menu and select Stretch/Skew. This seems illogical as you actually want to shrink it but you can achieve this in the dialogue box that appears by specifying the size to "stretch" by as something less than 100%.

Once the image is resized remember to use the Save As command under the File menu to save it to a different file name otherwise it will overwrite your original image.