Want to resize an image in 640:480 (640:400 is not a regular size, but it is also possible) pixels ?
It is also easiest in Paint. I am describing :
1. Start > All programs > accessories > Paint.
2. Click the resize button
3. In the upcoming Resize and Skew window, select resize by "pixels" , and give the pixel size of tour choice.
Click on ok. then save as jpeg image.