using vista? try to change to windows 7 64 bits if you can (or make a partition to install windows 7 just to test) win7 is the best microsoft operating system at the moment you will have better performance and compatibelity

i tested that "warez" version of cubase 5 with asio4all drivers on windows 7 64 bits and i dont remember having any problems

try using steinberg audio drivers too to see if it solves that problem (i dont remember the name of the drivers but i know cubase have is own audio drivers, check cubase audio options)