I suspect that the Microsoft Billing system for an online purchase of the subscription can't handle anything other than the 50 states. The US Virgin Islands would be considered International. You can either contact support for help and confirmation of this, or you can purchase a retail copy of OneCare (Amazon.com usually has it for much cheaper than the subscription price, even with shipping considered) and use the retail key to activate the subscription.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve