NO INTERNET CONECTION TO SOME SITES

Hello,
Can anyone please help or advise me, My problem is I cannot connect to some sites on the net ie internet explorer, google,yahoo,etc. Other sites are fine & my E mail works ok
I am using windows xp and have 2 computers connected by a hard wired LAN to a netgear router.
Both computers have the same problem
I have been trying just now to set up a VPN without success,which may have caused my problems.
I would be grateful for any advice, Cheers, John