I am having a strange problem. I am connecting to the net via a router. From my main pc, on certain websites (eg www.tesco.com, www.richersounds.com) I get the response 'the operation timed out...' I did think it might be a DNS issue, but I get the same if I enter the IP address. If I ping the url it looks up the IP address ok but comes back with 'Request timed out' . On all other websites it seems fine. This is with both Mozilla and IE.
On another pc on the network it works fine, so I guess it must be a (security?) setting on my main pc, but I haven't a clue where to start looking. Any suggestions?