I would have installed vista last. I believe it uses a different boot loader than XP. unless you can boot up in a repair mode for vista, and fixmbr on it lol.

I am well triple booting. I have Vista on a 40GB hard drive, and XP Pro and XP 64 on a 120 GB. so far no issues with that.

maybe if you unplug your vista HD, and then reinstall XP, it might work better.