Actually, to suppose the issue is with Comcast might not be correct.
Let's see. If I understand correctly,
* You have a Comcast modem. Everything works fine if directly connected to the Comcast modem.
* Adding a router between the Comcast modem and your physically connected computer does not work.
* Adding a laptop connected wirelessly to the router does work if no security is enabled.
* Adding a laptop connected wirelessly to the router does not work if security is enabled.
Okay, so let's first look at your desktop.
Do you have a firewall enabled on your desktop? If so turn it off and report what happens.
On your laptop, Be community minded and leave security disabled so other's can share your connection. Be sure to install a firewall like Zone Alarm, though. Figure if a "bad" guy wants to share your connection, he already is -- there are loads of sniffers out there that will discover your password.
--Mark
http://www.marksatterfield.com