1. What version of XP (original, SP1, SP2, SP3)?
2. What OS are you booting into to look at the drive? Really both? Then in Vista go into disk management and see what it reports about the partitions on that drive. See http://www.windowsreference.com/windows-vista/how-to-use-disk-management-in-vista/
And if you take the drive out and mount it in an external case, what does disk management say then?
It's too early to discuss data recovery yet. But it might be necessary.
Don't you have a backup?
i have a wd 320 gig for my secondary hard disk. on my primary, i partitioned it and installed xp and vista, but i took my wd out of my dasktop before the install. my os's installs went fine but when i put the wd back in neither os will access the drive. it askes me to format it and only shows 127gig available, and thats how much room i had left on it, help me get my stuff back... thank you