Edit: To clarify, just be sure your computer can handle running Vista easily, otherwise it may stuggle when you run Guild Wars as well. Slaanesh knows you'll be running other programmes at the same time as well.
Also, for clarification, the GW box doesn't expictly say that it works with Vista, because Vista didn't EXIST when GW first came out. It does work with Vista though.
I can run my game on Vista just fine. I turn off any unnecessary background programs (such as the windows sidebar) to improve performance and reduce risk of crashing.
i would also like to add that vista works, but either make sure that vista came with your computer or that you have enough memory to handle vista as well as GW