Can't help. I do agree with you it does seem to be a bit high. You should be getting about 488 gig. Perhaps someone else can explain where the other 23 gig has gone. The only thing I did think of was if it was an external drive, some come with software which is preloaded and might take up some space.
I've just read your post, and I seem to have lost 2Gb on my 80Gb hd. This is good to know though, because I posted the other day about increasing my hd size with Xp, and to whether I would get any problems with Xp recognising the larger size, but if I may loose a few Gb I'll have to allow for it, as I have alot of photos, and a few copies of my dvds on it.
To find out "where your gigabytes have gone" open Computer Management (RIGHT click My Computer) and click on Disk Management. RIGHT click on the drive you are interested in and click Properties. You will then see the drive capacity in Bytes and GB.
Example, I have a drive here of 81,948,430,336Bytes which is also listed as 76.3GB. It is a nominal 80GB drive.
The confusion comes in because the HDD manufactures tend to use the decimal definitions Kb 10^3, Mb 10^6 and Gb 10^9, whereas computer memory has traditionally been measured in binary so Kb 2^10, Mb 2^20 and Gb 2^30. So the Gb's aren't lost just hiding.