Not sure what you bought online, but I'm going to take a wild guess that you mean Windows Live OneCare.
if I'm correct, go to
http://onecare.live.com and download and install the trial. Once it is installed, open OneCare and click subscribe, then sign in with the LiveID used to purchase your subscription.
If you login with a different LiveID, you will be prompted to purchase a new subscription.
If you don't recall the LiveID you used for your subscription, you will need to contact support who can provide that information to you using your contact information.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
-steve
Microsoft MVP Windows Live / Windows Live OneCare & Live Mesh Forum Moderator
