if the belt is squeaking, for a cheap fix for a little bit, take a peice of lumber crayon (3 bucks at home depot) and run the car and rub it on the ribbed part of the belt, it will stop the noise for a little while. BUt watch your hands!!!!
It is the serpentine belt. I had the same problem just two days ago. I had a leak in intake manifold (or gasket...whatever) and I guess it leaked onto the belt and when they cleaned it, it started to squeak when accelerating. They replaced it and now the squeak is gone. The belt cost $77 CDN for just the belt. They covered the labour under the warranty work for the intake.
If the belt doesn't do it, it could also be the belt tensioner is no longer applying the right amount of resistance. I had to replace that after replacing the belt itself failed to solve the problem, although I had more of a chirp than a squeak.