Wireless router help needed please

I am after some advice to help my parents. They have Virgin cable BB via a NTL modem and would like to have wireless (mum has a kindle now)
I have a spare Linksys WRT54GL with DDRT firmware and thought I could simply plug the modem into the router. I have tried this and it basically doesn�t work. The PC can only see the router web page when the modem isn�t connected to the router.
Their PC is so slow to restart that trying different things and restarting takes so much time I thought I should ask for advice here ;
Does anyone know how this setup should work? I imagine it should?

Sounds like you've inadvertantly conected the cable modem to the LAN side rather than the WAN side of the WRT. If you're connecting it to what looks like the correct port then it could be that the config in DDWRT is such that the WAN port is actually configured to be part of the LAN network.

I see, probably best to reflash the router back to linksys firmware then use the WAN port as suggested.

Thank you.

All sorted now thanks. Reflashed the router to linksys firmware and connected as advised. It does take a while to pick up the various settings from Virgin though. I think I was being impatient.