It could also be a worn-out switch in your mouse.

Click to expand...

I've had mice that do that. It is the switch. This is called a "micro-switch". Here's a replacement:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=370198562470

This particular one comes with a bar across the top. This would need to be pulled off. These same switches were used in Apple UniDisk 3.5" disk drives.