I recently installed a router at home. And switched to cable.
Turns out it was MTU that was my problem. Because I could access many other internet sites, but yahoo! mail would return a blank screen.
Setting the MTU on the ethernet interface on my PC fixed this problem. I had to set my MTU to 1490 bytes (Telstra Australia BigPond ISP limitation). Other people have to set it to 512. (Note this is an advanced task in windows, you have to google around for a while and try and work out what it is in the registry you need to change to force the MTU to be set to a lower value).
Note that setting the MTU in the router wasn't sufficient.. it was my PC that had to have the MTU changed (I was running Windows 2000).