It takes a little time for the ISP to update the MAC address of the router instead of the PC you are using so that may have been one reason for it not working straight away.
To get your desktop working, it needs to be plugged into the router with a network cable. (RJ-45 on both ends)
Then you should run the network set up wizzard as it may have been connected via USB previously not via the network, which means it won't work! To run this: Open My Computer, on the left there is a link to My Network places, click this. On the left there is a link that says Set up a Home or Small Office Network, click this. Just go through clicking next until you get to the Select a Connetion Method page, (screenshot below). You need to select, this computer connects to the internet through a residential gateway... Further on in the wizzard you have the option of enabling or disabling file and printer sharing, do whatever you need. Once you get to the end of the wizzard it should scan the network and detect the DHCP settings from the router and work!