Well, this has been happening for a while, and it's just started to bother me [closing down internet windows while I'm in the middle of something]. For no reason that I can see, the mouse moves position, for example:
I'm clicking on the scroll bar to move up and won the page, and whilst clicked down, the mouse moves upto the "X" button and closes the window.

I'm just wondering if it's a common problem, and what I can do to fix it. It's not wireless by the way. Thanks in advance.

that sounds like a script , hack
run spyware programs and virus scaner

All optical mice "wig out" on occasion. It's usually attributed to dust or hair on the optical input. Blow it out and see if that helps.

Just as a warning....dont look into the mouse light while it is on.....turn the computer off....My laser mouse doesnt emit a light....the laser is on...so best thing to do is turn off the mouse or the whole system.

Hair it was, thanks guys.
*Looks into mouse light "ahhhhhhhhh"*.

The laser is not powerful enough to burn your retina's but still, I always heed Pain's warning. It's just like you never look down the business end of your pistol, unless the gun is dismantled for cleaning or maintenance.