|Boot off a Windows XP CD, choose the second repair option, the Recovery Console, select your HD at the prompt, log into it, and run "chkdsk c: /p" replace c: with whatever HD it is you wish to scan.|
There is a bug in NTFS under certain circumstances that will report your HD as full after a crash. Running check disk at boot up time, or from a bootable win XP cd will correct this error.
99% sure this ISN'T your issue, but I have run into it several times with missing HD space.
If this is not your issue, start at root (C:\) and see which folders are taking up all the free space, you might by chance find a folder with a corrupt text file in it being mis-reported as taking up your 500gb's. For instance under certain circumstances an old HP app called SNMP.exe, installed from the factory on HP systems, would keep on crashing and relaunching itself, it created an error report at every crash, several crashes in a few seconds and it added up to a directory with thousands of log files equaling something like 20gb's of junk space.
PowerMac 9600(1 ghz G4)
ATi 9200 PCI