The catch is that Vista will install it's boot loader (and a \Boot directory containing several files
) onto the primary *active* FAT/NTFS partition on the first HDD present in the box.
I have a 1Gb partition at the start of my first HDD upon which I have FreeDOS (hey, DOS was my first OS and I still have lots of stuff for it
). The RC1 trial of Vista pushed FreeDOS's kernel out of the way to make room for it's own bootloader. I was able to move the bootloader and the \Boot directory onto the partition with the rest of Vista and then boot it with LILO. (and if you want to know, I had to use a system floppy to restore my FreeDOS kernel).
So, at a guess, Vista can do damage on a system which currently has XP on the primary active partition of the first HDD. You'd probably have to use the Repair facility on an XP CD to allow XP to boot, then boot from a Linux CD or floppy and re-run LILO (or grub-install) to get back to a dual booting Linux/XP box.