Sounds like some kind of firewall or policy issue than a router issue.
What happens when you pull the router out, and connect directly in? Can you get to the sites then?
If so then have to suspect some sort of security setting within the router - though wouldn't explain why it doesn't happen with the other PC and laptop...which is why I think it's with your PC.
Maybe the security setting of your internet explore is set high enough to block those particular sites....
1.) have your, or do you currently have a firewall on this particular PC?
2.) what is the privacy setting of internet explorer?
3.) have you tried adding the sites as a trusted site and accept cookies in internet explorer?
- i.e. if its www.yahoo.com...go to internet settings, select Security, select sites (in the trusted sites location), and input the sites URL. go to the internet settting, select Privacy, select sites, then input the sites URL and select Allow cookies.
Need a lot more info to hlp you figure this one out.
D