I came across that a few times working in internet tech support. To be honest with you, I don't know a lot about the cause of it, but I usually managed to fix it by resetting the TCP/IP stack.
All you need to do is perform as admin:
Code:
netsh int ip reset out.txt
out.txt is just a logfile. <a href="http://support.microsoft.com/kb/299357">Here's</a> some info about netshell. Management used to tell us you could break stuff by resetting the stack, but I'm pretty sure they were just covering their asses. I never had a problem with it.
You probably should check device manager too, you may have corrupt drivers if you're using a USB router.