If you are running Windows XP and ZoneAlarm Firewall, Windows Security update KB951748 when load will render your computer unable to connect to the Internet.
I know. The last thing I did this morning before leaving the house was update my home computer. This evening it would not connect to the Internet or get email
I tried a different Cat 5 cable between my computer and hub. I tried different ports on the hub.
I contacted my cable provider who made aware of this new development.
In ZoneAlarm, if you place you Internet Security to Medium you will be able to get to the ZoneAlarm Homepage where you go to Technical Support and there is a temporary fix for the problem.
After the fix, you can take the Internet Security slider back to high. I'm typing this on the effected computer following the fix.
It doesn't seem to be effecting Vista Users.