Asus M2N-SLI Deluxe, NVidia RAID, new Win XP, and "unknown partition"

I couldn't for the life of me decide whether to post this in Storage, Motherboards, or Homebuilt, since each of those applies! Don't fault me for tossing a dart and picking Storage.

I have an existing system built around an Asus M2N-SLI Deluxe motherboard. I've been running Windows 2000 all these years, though I finally bought Windows XP (+SP2) last year. I have a RAID 5 array defined using the onboard NVidia RAID chipset, using four Hitachi 250GB SATA II Deskstar drives. That array has been fully visible and functional in Windows 2000 from the beginning. There are six SATA channels onboard, and I have all of them populated:
1: WD 1TB Caviar drive;
2-5: Deskstar drives as RAID 5;
6: WD 750GB Caviar drive.

Today I finally managed to successfully install Windows XP, after resolving the problems I've been encountering for weeks: XP Setup would hang during device discovery, and then on every subsequent reboot and attempt after that I'd get a BSOD on re-entry with Stop error 0xD1. I used a custom XP DVD slipstreamed with SP3, the NVidia SATA RAID drivers, and NVidia video driver. However, even though XP recognizes the RAID array and the 750GB non-RAID drive, neither of them are actually accessible: Disk Management declares both the RAID array and that 750GB drive to have "unknown partitions" and refuses to even let me assign drive letters.

Again, both the RAID volume and the 750GB are accessible in Windows 2000. What the heck have I done wrong or overlooked that prevents them from being accessed in Windows XP?