Help how can i configure my existing vista x64 on boot camp?

on my MBP 17, i just installed Snow Leopard and on boot camp Vista X64 installed, just bought parallel desktop for MAC 4.0, but i can' make it works, can somebody teach me how can i boot vista (already installed on boot camp) from mac using parallel? i tried every way, but still can't make it work,or do i have to delete or remove my existing vista on my boot camp first and re-install the vista through parallel? please i need your help. thanks