hopharrigan,
Since your comp is an old 800Mhz CPU the problem could be the mobo itself. Some mobos had BIOS Revisions that allowed the Mobo to see Drives greater than 137GB. You could start by looking there.
With the new Hard Drive, it might be worth doing a fresh install of WinXP with SP2 already Slipstreamed.
If your mobo is the problem, I would recommend a PCI IDE Controller Card. (I am guessing since this is an older Mobo, this is an IDE Drive and not an SATA Drive.)
Specifications:
Ports: 2x Ultra DMA/ATA100
OS Support: Windows 98/ME, Windows NT4.0, Win2000, Windows XP, Linux (Turbo, Caldera, RedHat, SuSE)
Features: 32bit/33MHz PCI; HighPoint HPT370 UDMA ATA100 Controller; Bootable disk support; Large LBA support capacity exceeding 137GB
This card only supports hard disk drives, not ATAPI devices (like CD-ROM drives)
Retail Box (see pics for details)
Take Care,
Richard