This sounds like an htaccess problem that I've had with a joomla site and virtuemart.
The whois says you're with 1&1 and I am unfamiliar with the way they handle redirects, but check to make sure that all requests with or without the www point to the www.yoursite.com address. That is, in the address bar, if someone types yoursite.com that it automatically points them to the full address of www.yoursite.com and not just yoursite.com
I host with godaddy and its under their site redirects. If you're using cpanel I'm not sure how to do it, but I'll bet someone on this board or at 1&1 can help.