If you have a subscription to OneCare, and it is only installed on your desktop PC right now, you can install it on up to two additional computers. So, on the laptop, remove any security software that it was shipped with such as Norton or McAfee and go to http://onecare.live.com - install OneCare from there. After it is installed, activate OneCare (subscribe) and sign in with the same LiveID that you subscribed on the desktop with. Open OneCare, click on Help/About... on the desktop PC to see the Subscription ID - that's the LiveID you need to activate the second install with.
-steve