Sorry. Activate onecare, gives me 2 meanings. Is onecare installed but not activated or subscribed on your computer? or are you still in the process of installing onecare?
Try this link for general steps for troubleshooting installation of onecare.
How to troubleshoot Windows Live OneCare installation issues
http://support.microsoft.com/default.aspx/kb/910656
To activate or subscribe onecare on your computer
Open main screen of onecare.
Click on subscribe button.
Sign in with your Live ID. Your Live ID is the email address you associated your subscription with.
To check which email address your subscription with onecare is associated with.
Sign-in to https://billing.microsoft.com.
You can also contact support for further assistance.
How to reach support (FAQ) -
http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
