If your are using XP, uncheck in your network connections...
IEEE / smartcard security
XP firewall
Check the router, if you use security: ip routing, mac addy authentication, etc.
In internet options,
make sure you have HTTP1.1. w/ proxy, use ftp passive firewall checked.
Lower the security zone settings also until you sort things out.
you can ping the router but not other pc's....your router is fine.
Also, if you set auto dhcp and dns to manual, set them back to auto for now