Defrag doesn't work

After functioning properly since I got a new computer with 64-bit Vista
nearly a year ago, the defragmenter now does its "spinning" for about 5
minutes and indicates: "Your file system performace can be improved. It is
recommended that you defragment now."

When I click on "Defragment now..." it spins again for about 5 minutes and
repeats the same message. This repeats each time.

Any advice?
Thanks, Alex