Dr TCP is a useful utility, but changing all the MTUs might not do it - especially if there's a router in between.
parabellum - what's the DNS address in your ipconfig /all?
You can check it (and a whole load more of this stuff)
here. That's a useful way to find out what's going on.
Ipconfig also has some utilities to clear out stuff - every so often, I have to type
ipconfig /flushdns at my laptop to get it to work.
I have no idea what's causing it, but it's quick and easy to fix like that so I don't bother.