If anyone's upgraded to MS Internet Explorer version 7 recently and are now having problems with websites timing-out or being reported as unavailable try reducing the MTU setting of your network connection.
I found that with an MTU of 1460 I had problems (having already reduced it to this to solve an MS Messenger problem) and had to reduce it further still to 1400.
A useful utility for setting the MTU value can be downloaded free from http://www.dslreports.com/drtcp