This confuses me..My first thought is, it's a 500GB hard drive, which would give you around 465GB. To my knowledge there hasn't been any 550GB HDD's made..(which would give you 500GB HDD space.
It's possible some 3rd party program was installed that displayed the drice in decimal instead of binary..if anything would do that.
Maybe someone else has been using your computer and messing with stuff?
If you take a look inside, the hard drive will say how big it is..if it's a 500GB, then you'll get 465GB. I know, I hate the fact manufacturers do this too.