You are running XP you said..pro or not this should work,
Click start, run, then type diskmgmt.msc into the run box and push enter to vring up disk manager.
I say this because it MAY be a 500GB drive, just the image the company uses may have only been set for a 250GB drive and so only made a 250GB partition, see?
see if there is any extra space and if it is format the spare space into a partition and they will = your 500GB.
(by the way, you don't really lose space
It's just that the manufacturers definition of a GB is 1000KB and windows definition is 1024KB
so you aren't really losing spae, you just never had it to begin with