Try the following:
1. Set Internet Explorer to be your default browser, if it isn't already, via Program Access and Defaults from Control Panel.
2. Open IE and in Tools/Internet Options, verify that your language setting is correct for your country.
3. Go to http://account.live.com and verify that the location for your LiveID is correctly shown, including country.
4. Go to http://billing.microsoft.com and, if you have a Billing account for that LiveID, check the contact information there, too. If you don't have a Billing account, ignore the message about no Billing account/relationship.
5. Delete cookies and temporary Internet Files from within IE.
6. On the IE connections tab, make sure "automatically detect" is not selected for the LAN settings.
7. Close IE.
8. Open OneCare and proceed with activation.
If the above still fails to bring you to the form for payment in your (supported) country, please contact support.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve