Need help figuring out router problem...

[weapon]

Here is the setup at my moms place.

Comcast HSI > Comcast Box for telephone > To linksys wireless g router.

Everthing works fine but I need to switch out the router and move to my brothers house.

I have an older linksys wireless b router that should work perfectly in the spot but whenever, I hook it in, all the lights go but it will not connect to the internet.

Then I switch the G router back in and it all goes back to noramal function.

Help...