Caviar Green 3 TB drive in Windows XP, recognizes only 800 GB

Windows XP 32-bit doesn’t support hard drives greater than 2.17TB.  But more importantly, it’s almost certain that your BIOS and SATA controller don’t support larger drives as well.

You get get a compatible HBA (host bus adapter) that might allow you to partition some of the drive, but the MBR partition scheme can only address 2.17TB and Windows XP 32-bit will refuse to recognize GPT-partitioned drives.  In addition, your BIOS probably can’t boot a > 2.17TB drive.  Usually UEFI motherboards are required for that.

The best thing about computers is that nothing’s impossible, and there are certainly ways to work around the issue you’re facing, but you are looking at an uphill battle here.

You’re better off with Windows Vista, 7, or Linux if you want to use an internal drive larger than 2.17TB, but Windows XP 64-bit edition is the oldest version that’s designed for these larger drives.