It sounds like a problem with the master boot record and partition table. Don't let XP initialize the drive, this would overwrite the partition table and all of the file allocation information.
Can you see the drive in a DOS prompt?
If I'm teachin egg sucking sorry!
Click Start->Run-> and type in 'Command'
then just type in the drive letter e.g. 'F:' of the drive and see if you can get a directory listing. (i.e. type 'DIR' and press the enter key).
If you can see files, use the XCOPY command to copy them to another drive.
If this doesn't work then you are at the mercy of data recovery companies that can rebuild the missing info on the drive. It is worth trying your local computer maintenance places as they are often much cheaper than the big commercial guys.