Jim's answer is correct if you are being presented with a renewal form or purchase form in the browser when trying pay for OneCare and only uS is shown for the address fields and remittance information.
If, however, you initially activated with a key and you have a Billing account for this LiveID at
https://billing.microsoft.com and the address shown for your account is US, I don't think that you can change it. Your best bet is to abandon the account and use a different LiveID to subscribe to OneCare.
-steve
Microsoft MVP Windows Live / Windows Live OneCare & Live Mesh Forum Moderator