Are you using a laptop with a touchpad? Chances are good that you are accidentally touching the touchpad while typing and that is moving the cursor. Go to Start / Control Panel / Mouse and see if there are any controls for adjusting the sensitivity of the touchpad or for turning off the touchpad while typing. Go to your manufacturer's web site and download the latest device drivers, firmware and software for you touchpad and your mouse (if you also have a mouse - you may need to go the the mouse's manufacturer for those drivers if it didn't come with the system). The new upgrades may provide options that didn't exist before like the ones I mentioned.
This happened to me as well and I bought a separate mouse and disabled the touchpad and the problem went away - but it had me going for a while until I finally figured it out by typing differently (with my hands raised higher so the palms didn't touch and had no problems). Even if this isn't the problem, updating the drivers, firmware, and/or software may on its own resolve the problem.
If none of the above works, we'll either need to run a full software troubleshooting set of procedures (post back and I'll reply with them) or the touchpad or mouse is defective and needs to be replaced. You can discuss the latter with the computer manufacturer when you talk to them (if you talk to them).
I hope this helps. If you have any questions along the way, post back and I'll do my best to answer them.
Good luck!
Lorien - MCSE/MCSA/Network+/A+ --- If this post helps to resolve your issue, please click the "Mark as Answer" or "Helpful" button at the top of this message. By marking a post as Answered, or Helpful you help others find the answer faster.