I don't know what account number you are being prompted for, but it may be the one found in Help/About in OneCare.
If you are unable to login with your LiveID password, and it tells you it is invalid, it is. If it throws another error, it could be a server problem. In any event -
Unofficial path - send me an email at sboots@mvps.org with a subject line including "OneCare" (to get past my spam filtering) and in the message body, tell me what country you are in, your forum nickname, and paste this link: http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=4197092&SiteID=2
I am not online 24x7, so you may be waiting for a reply, but I'll do my best to hook you up with support.
-steve