Squealing fan belt

First thing I'd do is verify the noise is from the fan belt and not from something else. If the squealing is coming from the belt hand spin the pulleys to make sure there isn't any bearing noise, a mild noise in a bearing can turn painfully loud at a high speed, and double check the deflection of the belt.
Do you have a manual or auto transmission?
Is there anything else going on when the noise happens? Shifting? turning?
Are there any other mechanical/drivability issues?