:O damn
Check the mouse's settings in the system control panel (Start > System Control Panel > Mouse on Win XP, dunno what you using)
Some Mice who got there own drivers have manual settings for their keys which might be corrupted.
Also check for problems in the 'hardware'-tab.
If that doesn't help check the device manager (start> system control > System > 'hardware'-tab > Device manager - button)
and check for your mouse, either if it's displayed correctly, and if there have been any hardware related problems.
If no problems show up i'd suggest updating or reinstalling the driver might help,
the last solution might be getting a new mouse.
Hope i could help. Post your experiences during trying these things up there ^, might help solving this.