Help cursor keeps going back to the beginning of what i am typing

I am beginning to hate this computer the cursor keeps going back to the beginning and the widget that displays the month and day is going crazy the months keep going backwards and don't stop. I can't log into anything without having to fight to get the cursor to move forward and on the desk top the cursor seems to move uncontrollably. I do not click on anything and i see it move from right to left . The drop down boxes move also fast across the screen and I don't even click on them is my computer affected by a virus? DO i need to reinstall Tiger again? The system is running on tiger and I am tired of the pounding sounds and fighting to try and keep the cursor from going all the way back to the beginning no matter if I hold down the spacebar it constantly tries to move back and won't move forward without fighting it