If you have PartitionMagic, it comes with something called BootMagic. It will present you with a list of OS's that are found on your system and give you the choice of which to load.
My dad used it when he had 2k, xp, and ME installed on the same machine.
I don't know if any of the free boot managers that linux users use will be any good for this, so I can't give any advice on them or even say which are any good.
Someone here with more experience with those could probably tell you which would work and be best.