If you are familiar with the Command Prompt, there are a few things that you can do from the command line, that could give you some clues.
First thing i'd do is to get the detailed IP address information for both computers, by running the ipconfig command:
Click Start >> Click Run >> type cmd.exe >> Press Enter
Once you have brough up the Command Prompt, what you are looking for is the detailed IP address information, you want this from both computers. Issue the following command:
ipconfig /all >> Press Enter
This will output the IP Configuration information for all your NIC devices that are present on your computer; you can write this information down, take a screen shot of it, or redirect the output from on-screen to a text file as in the following:
ipconfig /all > %filename%.txt
In this case %filename% is a variable which you can name whatever you like. NOTE: when you redirect [>] the output of the ipconfig command, it will save the file into the directory that is present on the command line, which by default is C:\Documents and Settings\%username%, where %username% is your login username.
Do the same thing on both / all computers. This is so you can make a comparison between all machines to determine if in fact there is an IP Address Conflict or duplicate IP addresses between your machines. Pay attention to the following:
* - Dhcp Enabled (may OR may NOT be set on both machines, this could be by design)
1 - IP Address (should NOT be the same on both machines)
2 - Subnet Mask (should be the same on both machines - unless you have subnetted)
3 - Default Gateway (should be the same on both machines)
4 - DHCP Server (should be the same on both machines)
5 - DNS Servers (should be the same on both machines)
You need to confirm that the IP Address | Subnet Mask for all machines is NOT the same, that would indicate an immediate problem - in a single network no two machines should ever have the same IP address.
IF DHCP is enabled, you know that the IP addresses are being automatically assigned, and should therefore, be relatively close in numerical sequence. If you see a huge gap in the numerical sequence, there is a possibility that one of the machines is configured with a Static IP address while the other may not be. The problem with configuring Static IP addresses, is that if it is not reserved in the DHCP server, there is the possibility of manually assigning an IP address to a computer to which the DHCP server has already assigned the address, resulting in 2 machines with the same IP address.
If all machines have DHCP enabled, you can release and renew the IP addresses on all machines to rule out the possibility of duplicated DHCP IP address leases as follows:
ipconfig /release >> press Enter
ipconfig /renew >> press Enter
If you have one or more machines that have DHCP disabled, you need to determine if this was done as part of the design of the network; meaning if you wanted or needed to use Static IP addresses, then you would not want to use DHCP OR create DHCP reservations for the Static IP addresses which your machines were going to use. This is a little advanced and probably impractical for a SoHo network.
Since you have the network information on both your machines, you want to test network connectivity and transmission next - just because you can't get to the Internet, doesn't necessarily mean that you aren't connected to the network. So issue the ping command from one machine to the other and see if they are talking to each other, as follows:
Where "ipaddress" is the IP address of the machine that you want to send a ping packet to.
If both machines are talking to each other, you should get a response without loss of packets. If you can confirm that both machines are talking to one another, then you want to ping your router or modem to ensure that it is talking to your machine - most likely the answer is yes if you got this far, the router is the Default Gateway and that should be issuing out the IP addresses and allows your machine to connect to the Internet. In my experience Linksys WRT45G routers are usually setup with IP address 192.168.1.1 and assign IP addresses to connected clients in the range 192.168.1.2-192.168.1.254.
In any case, the IP address of the Default Gateway is the IP address that you want to ping in order to determine Internet connectivity. Again, you may be able to ping back and forth from two machines that are which have network connectivity, but are still unable to get to the Internet because traffic may be blocked or dropped at the Default Gateway or Router.
I'd start with those steps first; try it out see what results you get and provide some feedback for ideas on what to do next.