USB port not working for mouse

My four-year-old Dell Precision M4300 laptop running WinXP has only three
USB ports. I use the one in back for the printer, which leaves two on the
right side--one above the other.

My Contour three-button mouse works fine in the top port but has never been
recognized in the bottom one. Other devices--external hard drives and flash
drives--work normally in the bottom one. I'd prefer to have the mouse
plugged into the bottom port because it's difficult to plug in the other
devices under the mouse connection.

Any suggestions for troubleshooting and fixing the problem?

Thank you!

Jo-Anne