If you are being prompted to buy OneCare during activation on your second PC you are likely not using the *same* LiveID as you used on the 1st computer. Open OneCare on PC1 and click on Help/About to see the Subscription ID. That is your LiveID that you need to activate with.
f you don't see the subscription ID on PC#1 and 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 Key and/or contact information.
How to reach support - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
Alternatively, sign in at http://billing.microsoft.com with email addresses that you have set up as LiveIDs and one will show your subscription listed - that would the LiveID to use to activate the additional computer install.
-steve
