Some Internet sites work

I have a enw XP Pro machine that can access some Internet sites, but not others. Connection is DSL via LinkSys firewall/router. My other PC (2000 Pro)connected the same way can access all sites. I’ve tried using different browsers (Firefox, IE), different ports on the router and even changed out the router. Name resolution works fine, but the connection times out on the affected sites. If I direct-connect the PC to DSL it works, so I would tend to blame the router, but my other PC is fine. I have run out of ideas….