You might need to check your brush settings. Make sure that they are set to default.
You may also want to go into your preferences and try changing your display/cursor settings. I'm not sure that this will do anything, but its worth a shot.
If this does not fix your problem, I'm afraid I can't help you
It sounds like there may be a corrupted file somewhere, but you said you re-installed so the probability of that is virtually nill.
Has this been a problem since you first installed the software?
Or is this a recent problem that has occured for no apparent reason?