A recent observation regarding Microsoft update.
Hotfix KB893066 causes network problems on certain Windows XP SP2 systems. Details on the patch are here http://support.microsoft.com/?kbid=893066
My exact experience with this update involved and IBM ThinkPad G41 running fully patched Windows XP Pro. After applying this update / patch networking functions were very very slow. In addition the issue that caused the research was the fact that any webpage requiring a login would fail after entering the username and password. The failure did not result in an error but rather a 'page could not be displayed' appearing to be a timeout situation.
Here is Microsoft's acknowledgment, or at least one of them.
http://support.microsoft.com/default.aspx?scid=kb;en-us;898060
I don't have any other specific information as to what systems are or aren't affected. I believe, however that this problem is isolated to XP systems only as I can not reproduce the problem on any Windows 2000 machine tested.