The instructions copied below need a step between step 3 and step 4, saying to close XP and boot into Vista. Then I installed EasyBCD also into Vista and only then would step 4 work. Now it works real nice. Thanks.
Setting up the Dual-Boot
Once Windows XP setup has finished, it'll automatically boot you into the newly installed copy of Windows XP – note that you will
not be able to boot into Vista at this point, nor will you see a boot menu option for it.
- Once in Windows XP, download and install the Microsoft .NET 2.0 Framework.
- Download and install the latest version of EasyBCD.
- Once in EasyBCD, go to the "Manage Bootloader" page, and select "Reinstall the Vista Bootloader" then "Write MBR" to get the Vista bootloader back.
- Once that's done, head on to the "Add/Remove Entries" page and select "Windows NT/2k/XP/2003" from the drop-down list, give it a name, then press "Add Entry" to finish.
- Reboot.