HollyA,
Here is an alternative way to install onecare on a 2nd or 3rd PC.
On the 2nd or 3rd computer
Go to http://onecare.live.com
Click on Free Trial
Choose to Install the Free Trial Version of OneCare
After sucessfully installing onecare, subscribe using the Live ID or the email address you used to activate onecare on your 1st machine.
To check what Live ID you have used to activate onecare on the 1st PC
Open onecare
Click on Help > About Windows Live OneCare...
The Subscription ID is the Live ID used to activate onecare on your machine.