Whatever raid metadata was written by WinXP-64 was destroyed and I was able to do a proper install of CentOS 6.
I'd still strongly recommend, if you haven't done so, check your BIOS to make sure your SATA controller doesn't have RAID mode enabled. I've seen the odd weird interaction between Linux, the BIOS, and the SATA controller (especially early non-"Intel ICH" family) that was traced back to that setting.