I recently put a 64 bit Vista formatted Drive into my XP HTPC box and found it was unreadable and could not be formatted.  Computer Management displayed the drive as a GPT Protective Partition. Unfortunately Windows XP cannot read or write this type of filesystem let alone mount the drive. A GPT disk uses the GUID partition table (GPT) disk partitioning system.

Your Options are:

• Run the DOS command line "cmd" in Start - Run
• Type DiskPart” at the prompt.
• “list disk” will show all disks on your machine and find the Disk number you want to convert.
• Change the focus to the required disk by entering the following “select disk X″ where X is the disk number you require to convert.
• Entering “clean” command to remove GPT disk from the current in-focus disk by zeroing sectors or if you are using Windows Vista or 2003 server enter "convert mbr"

or Using the Windows interface

• Back up or move all volumes on the basic GUID partition table (GPT) disk you want to convert into a master boot record (MBR) disk. If the disk does not contain any partitions or volumes, skip to step 2.
• For more information about backing up volumes, see Related Topics.
  Open Computer Management (Local).
• In the console tree, click Computer Management (Local), click Storage, and then click Disk Management.
• If the disk does not contain any partitions or volumes, skip to step 5. Otherwise, right-click the any volumes on the disk and then click Delete Volume.
• Right-click the GPT disk that you want to change into an MBR disk, and then click Convert to MBR disk.

Windows XP Disc support Windows FAQ.