Remark to Borkar's post:
XP will indeed overwrite the the MBR containing the Vista entry. So Vista will not start anymore.
But even if you install XP after Vista, you can restore the Vista-entry by either running the
Startup Repair-option from the Vista-DVD or using a third-party-tool.
I successfully tested EasyBCD. Just install and start it in the new XP (.NET 2.0 required), chose
Add/Remove Entries ->
Add Entry ->
Version ->
... XP ... -> give the entry a name ->
Add Entry.
Then go to
Manage Bootloader and klick
Write (make sure, that "Reinstall the Vista Bootloader" is selected).
From the next reboot on you will have a bootmanager where you can select Vista or XP.
Note1: Do not change the drive letter (should be C:), even though XP is most likely located on D: or E: as the boot-partition matters, not the XP-install-drive.
Note2: after the XP-installation you will no longer be able to access the Vista-recovery-partition. Back it up
before you install XP if you need it. Depending on the position of the recovery partition on the drive it may be a good idea to install XP in the (now useless and space-messing) former recovery partition, as you will lose no extra drive space.
Note3: The current version 1.7.x doesn't work with XP. The linked version 1.52 does. I have no idea if 1.6x works.
http://www.amilo-forum.de/download.php?id=3127 (size=1.61MB)
@aspettl: If possible, please replace this link with the standard-download-pattern or tell me how to re-link it without re-uploading the file.