What showing for your hhd size ia about right. My 120GB sata only shows as 114GB's. It is partly due to how they calculate the bytes size a GB isn't a 10000 but something like 10024 or something like that. and you lose some when setting up for some strange reason, I always seem to loose a 8MB into a very small partiton.
Concerning how xp reads the drive type, I can't seem to get into the hardware bit at the moment, but if I remember rightly my sata hhd also shows up as a scsi drive.
As long as it works I shouldn't worry about it.