Mouse pointer moving too fast suddenly

hi folks,

I'm using windows xp, and last Friday my mouse pointer started to move faster than normal, despite no changes on the settings. I tried to move the settings down and back one point, but either it is too slow or too (lightning) fast, instead of moving as little as I'm used to, it ends up at the upper right corner of the screen.
I tried to restart the PC, and after leaving it rest shut off for a couple of hours, it went back to normal, but then started again to behave strange (meaning it seems to be acting normal on the desktop, but speed up on web pages or word documents or games, although I'm not sure how much).
what could be the cause, how could I correct this? any ideas? past experience?

o.m.