How can I force a URL to have "www." no matter what?

I'm using Joomla 1.5 and Virtuemart 1. It might seem like a weird question but I need my URL to have "www." written in the URL at all times. If the URL doesn't have "www." then the cart functions don't work. If it does have it, then it does work.

Is there a way I can force this? I am using Parallels Plesk 10.3.1 if I can do it on there.