mouse has mind of its own?

I have a laser mouse but latly when i go to click or just put the curser on to a page it will go down to lower right hand corner or anywhere other then where I want it to go .Also pages pop up and menues .Ive scaned for viruses ,ran spy-bot It only found dso.and it says to have fixed that.My virus defs are up to date.it doesnt do all the time .Is it the mouse or some kind or worm?