|Does this happen with all applications or just one like your browser?|
Are you using a LED/Laser mouse or the old fashioned roller ball type?
If the latter, the ball probably needs cleaning as they do pick up a lot of gunge.
Can you scroll with the cursor key?
Mice are so cheap that if I couldn't solve the problem in a few minutes I would go out and buy a new one. That would eliminate the mouse as the problem. Even if it is not the mouse, a spare is always handy. Mice do not like having beer spilt all over them.