I've moved your post to the General topic folder.
As far as I know, the offer for 29.95 is only available via a special link from within OneCare during the trial. If your trial has expired, the offer may not be available to you. However, I do not know this for sure, so you may want to contact support via email as a trial user to confirm:
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
Note that even though OneCare is expired, your PC is still protected. You won't be able to make changes to OenCare settings or run manual processes, though.
You can also look into getting a retail copy of OneCare for less. Amazon sells it for 19.95 plus shipping. You simply need to use the key to subscribe - don't reinstall OneCare. The retail price from Microsoft is still 49.95 per year when activating or renewing directly.
-steve