I'll go through some basics for someone not too familiar with XP/PC's. I don't mean to be stating obvious stuff if you're familiar with them. Sounds like you're primarily a Mac guy.

First thing to do is find out if XP recognized the hardware. Right click on My Computer->Properties->Hardware->Device Manager.
Go down to Netwwork adapters and see if your ethernet port is recognized. If there is a red X or a yellow exclamation point over it, you need to find drivers for the device. Hopefully you have them on CD like Jonthebomb said.
If it says the device is working properly, right click on My Network Places->Properties and right click again on the Local Area Connection icon (there may be more than one). When you right click, go down to Properties and open it.
Ensure the TCP/IP check box has a check on it. Highlight it and click Properties.
Comcast uses dynamic IP, so make sure the Obtain an IP address automatically and Obtain DNS server address automatically are checked.
Just a couple of other things... If you are not using the machines on a router (just switching the ethernet cable back an forth between the PC and Mac), you need to unplug the cable modem when you move it between machines. If you don't, it will appear that you have an IP address, but you will not be able to surf.
Hit Start->Run and type CMD in the dialog box. When the DOS box comes up, type in ipconfig and hit Enter.
If you have connectivity to Comcast you should see an IP address (typically starting with 68. 69. or 71.xxx.xxx.xxx) and some info telling you what you're connected to at comcast.net.
If you have 0.0.0.0 or a 169.xx.xxx.xxx IP, you're not getting out.
Good luck!