Cursor Problems

I have a four year old Compaq nc6000 laptop and the cursor at times changes to a circle with 2 arrows and starts scrolling erratically. The cursor will also freeze up and I am unable to type and/or click on anything. At times, I have to power off the computer because I am unable to close programs and close down the computer. When I am working in Excel, the cursor scrolls uncontrollably. When I am not hooked up to my port replicator, I do not have the problem as much.

Also, the problem seems to be quite severe when the fan is cycling rapidly on and off. What the heck is going on?