I recently have gotten high speed comcast internet in my home and it works great on my home computer, but when I try to hook it up (by taking the ethernet cord out of my comp and plugging it into my laptop) to my laptop (Dell latitude d600), an error message pops up saying that there is limited or no connectivity. When I try to repair it by pressing the repair button on the pop up, it fails because it cannot renew the IP address (?)

I used to do this all the time with my old internet provider (at&t dsl) but now its not working. PLEASE HELP ME!

I am no computer geek but I answered because many times I get this on my Dell latitude D620. I am wireless though and it seems like it is random. I try a re-boot of my wireless router thaqt sometines works, a re-boot of my Latitude that sometimes works, a re-boot of my internet box too sometimes.
I am wondering if you shutdown your desktop then unplugged the cable and put it in the laptop before turning it on would help?
I would be curious to know. If not I can ask my IT guy for you in the morning

Do you have DHCP enabled on both the router and the laptop?

Bikehappy may not be a self proclaimed geek but his solution corrects most home network connection problems about 90% of the time. Rebooting your router and cable/dsl modem often corrects poor or no connectivity problems.

Well I tried to power down the box but it has a battery abackup system in it and won't turn off...

And I'm not sure what a dhcp is? Sorry

What it sounds like is your router(the box that comcast gave you) has only one IP address to assign. An IP address is like a phone number for your computer and it costs your isp to give out more than one. There are two ways to fix this.

1. Reboot the router when you switch the cable. This will reset the router and cause it to resend the ip address to what ever device is plugged in.
2. Go to your local computer store and get something like this Belkin - Wireless-N Wireless Router with 4-Port Switch - F5D8233-4 (http://www.bestbuy.com/site/olspage.jsp?id=1178925992300&skuId=8374476&type=product) This allows you to assign private ip address so that you can have multiple devices working at the same time. Follow the setup that comes with it should be pretty straight forward with the directions included.

Micheal's solution is a good one that will usually work. It's never a bad idea to pick up a router when you have more than one computer. The modem that the cable company gives you then communicates with the router, and the router communicates with all your devices. This will save you the hassle of having to unplug one to plug in the other.

To power down the modem, simply unplug it at the device itself and wait about 10 - 15 seconds. This is usually sufficient to let it completely cycle through. I just had a crazy thought. Are you using a networking cable or a USB cable to connect the modem to your laptop?

Example of an Ethernet Cable (http://cdn.overstock.com/images/products/L10726965.jpg)
Example of a USB A/B cable (http://bose.infopop.cc/groupee_files/attachments/5/2/0/5201064594/5201064594_usb_cable_a_b.jpg?ts=486DCFF9&key=E19D4C024BDAF8A1BAEB1280D36BB975&referrer=http%3A%2F%2Fbose.infopop.cc%2Feve%2Fforu ms%2Fa%2Fga%2Ful%2F6201064594%2Finlineimg%2FY%2Fus b_cable_a_b.jpg)

I ask this because if you're using the USB cable to connect your computer to your modem, you really do need to power cycle the modem each time you switch computers. Also, you need to make sure that you have all the software that came with your modem installed on both computers. If you're connected via Ethernet, you don't need any drivers for your modem, but if it's USB, then you have to have the drivers installed for it to work properly.