PC Opens The Same Program Over And Over Again With Single Double Click

Every once in a while my pc acts really nutty. What happens is that I'll go to open a program (lets say Nero Burning Rom) and on a single double click Nero will open up 70+ times. Furthermore, it makes mostly all of my programs act up when this starts happening. For example, when I click the arrow in the system tray it expands and collapses tons of times before it stops. If I open Firefox my homepage sort of half loads over and over and won't stop. MSN Messenger opens up then freaks out when I put the pointer in its window (this happens in the above Firefox example too). All of this activity makes the pc unusable until it all stops. Speaking of when it stops, it does't stop with a restart, it just stops mid session. I thought it could be a mouse problem because the trackball I use is pretty ancient and I figured it just might have just petered out finally, but then I tried another mouse that I found laying around and it did the same things. One other thing I've noticed it (the trackball) doing is when it's not being used the pointer slowly crawls towards the top of the screen. I've run my anti-virus and all three of my spyware programs and none of them find anything out of the ordinary. So what could be causing this? I'm starting to think it might be my mouse port, but how would I find this out?