Following installation of XP service pack 3 get

First of all - it is not a router related problem
When you install SP3, it adds a new service for your Ethernet and Internet connections, and often, if the driver is not up-to-date, it is not fully compatible with SP3.

The first step should be a driver update for your network card(s).
If you need help with that, let me know the make and model of your network card and we take it from there.

If that doesn't solve the problem, try the following steps.

1. Go to Start>Run and type "services.msc" Click OK.
2. Scroll to the bottom and you will see an entry called "Wired AutoConfig".This is a new feature with SP3 and needs to be enabled.
3. Right-Click on it and go to Properties.
4. There will be a Drop-down box where probably "Manual" is selected. This has to be changed to "Automatic".
5. You start the service by clicking "Start".
6. Close the window by clicking "OK".
   

If you have any more questions or this did not solve your problem, just come back here and leave a comment - I'm happy to help.