Changed boot.ini a bit and made it work. Installed Ubuntu 10.04 in place of CentOS and adjusted the partitions. THe 64-bit XP became the third partition on the second drive so I adjusted the boot.ini like thus:
[boot loader] timeout=30 default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Windows XP Pro" /noexecute=optin /fastdetect /usepmtimer
[operating systems] multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Windows XP Pro" /noexecute=optin /fastdetect /usepmtimer
multi(0)disk(0)rdisk(1)partition(3)\WINDOWS="Windows XP Pro x64" /noexecute=optin /fastdetect /usepmtimer
And voila! Everything works like a charm.