emptypockets,
Try to verify if the Live ID you are trying to activate onecare with is the one which have your subscription.
Sign in to https://billing.microsoft.com verify if the subscription is active on that said email address or Live ID.
If not, try to sign in using any possible email address you have that your subscription might be associated with.
If you got the correct Live ID but onecare is still not accepting it. Reset your password to 6 characters only then try to activate onecare again.
In case you can't remember the correct Live ID, contact support.
How to reach support (FAQ) -
http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2