The mouse moves errastically.

have you tried replacing the mouse?

If you have an Optical Mouse, say a Dell it doesn't like a smooth surface mouse pad. Try one of those rubberized ones.

You might also try a different USB port.

After ruling out the mouse hardware and port, I would look at adware or spyware as a culprit. Run Spybot Search&Destroy or equivalent program to see if there's any problems.

If you don't turn up anything with a Spybot scan (you probably will), and if a replacement mouse doesn't work correctly, try the following URL for a couple more possibilities

http://www.micro2000uk.co.uk/techtips/techtips_possessed_mouse.htm