I have two drives, a big SATA 200G primary, and another ATA 60G I threw in there as a slave. Simply booted off the XP CD, chose the correct drive, installed XP, and it was fine. Contrary to what I had read, it DID NOT replace the NTLDR and related files on the primary partition with their 32-bit counterparts (Although I'd still recommend backing them up somewhere safe -- that's boot.ini, NTDetect, and NTLDR).
The only remaining task was to edit the boot.ini file on the primary to allow for the dual boot. Mine looks like so:
Note that the key number is rdisk(1) in the second OS line. Rdisk(0) is your primary drive, then rdisk(1), etc. That's all there is to it! I have a 5 second timeout set so if I do nothing when the list comes up, I get my desired default OS.
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Windows Server 2003 Enterprise x64 Edition" /fastdetect /NoExecute=OptIn
multi(0)disk(0)rdisk(1)partition(1)\WINDOWS="Microsoft Windows XP Professional" /fastdetect
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