Open a browser window on your laptop. At the "address bar", type in "http://192.168.0.1/" without the quotes and hit enter, then it should bring up the router's user interface.
Btw, what router do you have? Different routers have different interface designs. But anyhow, look under "Advanced Settings" or "Advanced Network" tab. Looking MAC Address Filter. If it IS in fact, set to enable to filter out MAC addresses that do not belong on the network, you'll have to obtain the desktop's MAC.
On your desktop, go Start-->Run, at the space type in "cmd" without the quotes to bring up Command Prompt. If you see a directory name like this: C:\Documents and Settings\YourNameHere>
Then at the end of that line type in "cd\" without the quotes, hit enter, it should leave you with a clean C:\, and then type "ipconfig /all" without quotes, hit enter. Look for the Ethernet Adapter Configuration section, and "Physical Address" is the MAC Address of that machine. Write it down and enter that into the MAC Address Filter page on your router's user interface. Save.
These are for, IF a MAC Address Filter is setup. If not, let me know and we'll try other things.