Mouse cursor moves up and across the screen when the mouse is stilll

Using any mouse, (roller, optical or wireless) when the button is not held down, the cursor slowly works its way up the screen. Using WinXP OS. Have slowed down the mouse response bar but is no real help. Have tried Microsoft downloads but no help. Have tried both Microsoft and Generic Mouse but no help. All was fine until went on holiday for a month now its not good.
Any help appreciated Thanks, Norm