Can you go into the 'startup and recovery' screen in the system configuration and select the option that allows you to selet the OS you want?
SOrry i'd walk you though it but i'm on my vista machine right now. But go into your system properties, then go to startup and recovery and you should have an option there.
If it doesn't allow you to choose or it only recognizes XP (since vista is on another drive) you'll have to use a 3rd party tool.