To activate in Belgium your IE language needs to be French or Dutch. It can then be changed back. It's a rather misguided effort to prevent purchase of OneCare in a country where it is not available. Your IP address is checked against your IE language. This check does not happen if you install to a 2nd and 3rd PC and activate with your same LiveID.
See this post regarding Activation redirecting you to a page where you cannot activate, but are presented with a view of available Windows Live Products instead -
http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=1202298&SiteID=2
-steve