Not enough information to tell for certain, but enough to get started
XP detects the drive as a GPT protected partition, so that it won't accidentally do nasty MBR related things to it since the drive appears to be GPT based.
We don't know the filetype - the 'easiest' way would be to try to mount it on a linux livecd and check, maybe using unetbootin - the simplest way would be to use df -t in a terminal or You could use gparted for a graphical interface. If its NTFS, and its some random glitch, sometimes you can recover the data from linux and reformat.
Assuming we need to check in windows, and its an unsupported filetype, a good bet is HFS+ since OS X uses GPT by default (plausible, since most hard drives are still MBR formatted) you could check with HFS Explorer if its that, and what's there.