Open the OneCare GUI and click the Activate Button you see there. When it asks for your LiveID, enter the one you used to subscribe with. There was no reason to re-download the program, since it is exactly the same program version you already had installed.
However, if you actually re-installed OneCare from a 'new' CD, then you'll need to update your subscription first by loggin into billing.microsoft.com and selecting Renew using a PIN, then entering the 25 character PIN from the orange tag found inside the box, usually on the CD sleeve or on a small card for older packages.
OneCareBear
< EDIT > Jim, we have a stutter today, and you're always the quickest one.
Windows OneCare Forum Moderator