First, you'll need to install the NTL cable modem software on the laptop, just like you did on your desktop, remembering to plug in the modem into your laptop (USB/Ethernet) ONLY when asked to do so. This should setup the connection/modem to work on your laptop.
Whenever you need to use the connection with either desktop or laptop, just swap the modem's cable from one to the other, with the correct interface (USB/Ethernet) that was initially setup for each.
The above is how I've used my NTL cable modem with 2 independent PCs (both desktops). G