It may be the drivers that need re-installing or adjustments made to the touch-pad controls of the mouse pad.
Assuming your using XP go to your control panel, click on Printers and other hardware then click on the Mouse Icon. This will open a window with several tabs across the top and you should be able to make adjustments from there to all the actions of the mouse.
If this does not cure the problem, right click on My Computer Icon/Properties/hardware tab, click on the Device manager button and scroll down until you see your mouse device. Click on the + sign and your touch-pad will show up, choose renew driver from there, it should be on a floppy disk or CD that came with the PC.
Post back to this thread if you need any more help and someone will guide you further. j.