One aid in this situation would be to run PQMagic partinfo and see whats going on. Run it from a DOS 7 boot with and without some of the peripherals connected. Is the 8 Gb XP Pro showing a valid partition (e.g. one you had deficned?) If not, DO NOT let XP Pro write to that drive or whats already on it may be pooched.
Do you by any chance have a ZIP drive on this box?
That Seagate drive is ATA100 so this may not apply but let me relate a story. I had a maxtor IDE, a Seagate SCSI, and an IDE Zip drive all on the same system. I could not get NT to install correctly because it kept failing to see the actual drive configurations. correctly. It turned out that removing the ZIP drive solved the problem. After installing the system I then installed the ZIP drive and everything is good. ZIP drives identify themselves in a non-standard fashion which really confuses NT. Maybe this is happening to you. So as a first cut at doing this you might try disconnectiing non-essential peripherals, try the install, and install the peripherals after the OS is successfully seeing the harddrives.
AMD Phenom II X6 1100T BE Gigabyte ga-990fxa-ud3 am3+ 4x2GB DDR3 2xIntel X25-M G2 Samsung 830 & 850 Pro XFX Radeon HD 7870 BenQ FP241VW 24" Logitech G710+ & G400 SeaSonic SS-660XP 80+ Pt CoolerMaster Hyper 212+ WinXP64Pro Back-UPS NS 1250