How old is it? Try contacting Logitech for a replacement under manufacturers warranty? Although I have heard they're a pig to deal with getting replacement items or parts.
You could also check the Logitech forums as they might be able to give you better help than those here. Also go pick up a cheap £5 mouse on Amazon or something just to get you through (this is what I did when I had mouse issues).
@Turelus - EU PC Megaserver "Don't count on others for help. In the end each of us is in this alone. The survivors are those who know how to look out for themselves."