As long as your subscription hasn't run out or auto-renewed before you do it. It will add a year from the end of your current subscription so you can do it anytime before it runs out. I just did this myself with an unused 1.5 product key last week.
Go to the Billling site and log in using your Windows Live ID and password.
https://billing.microsoft.com/
Click on the Windows Live OneCare Service name with a status of 'Enabled'.
Click 'Renew this Service' (under End date)
Select to renew by using a Pre-paid PIN.
Enter the PIN and click next.
This should take you to a verification page displaying the new status and subscription end date. My PC indicated the correct subscription end date almost instantly, but was still indicating that the renewal would take place on the original date, I believe because it actually waits until then to apply the PIN, since otherwise it would renew as of the current date. Some say this disappears within 24 hours, but I didn't see it change since it was in another town and I had to leave.
OneCareBear
