Wow, you know you have done something wrong when you start shutting down your customers' systems. This is exactly what happened today when McAfee botched an update on their Corporate Anti-Virus application.
During a routine update labeled DAT 5958 Windows XP users suddenly found their systems unusable. It seems that this update was removing the svchost.exe file. Svchost.exe is a file that Windows uses to host different services that are needed to run windows. Without it you can (and will) get reboots, loss of network connectivity, inability to log on to the system and a bunch of other items.
McAfee has issued a fix for this issue, if you are one of those unlucky enough to have been affected. I feel for the IT staffers that will be working long hours tonight fixing any end user systems that have been taken out by this.