|Try tapping F8 before windows would normally start to load. That should bring up the boot menu but I think it will be the XP menu since that's your default OS. But give it a try and see if it allows you to boot to another OS.|
Or, if you hit the PAUSE/BREAK key at just the right time to pause loading at the OS selection screen you can select Vista at that point.
If you can get Vista to load then you need to run BCDEDIT or a third party application like EASYBCD to edit the entries.
If neither of those work you'll probably have to either do a fresh install or run BCDEDIT after booting from the Vista disk. From:
"BCDEDIT works from installation boot dvd too. If you mess up with a setting and you cannot boot, just boot from DVD and enter Repair Computer, then go to command prompt and there you go. You can play again with bcdedit to restore your system back."
More detailed info here: