So I try and dutifully run Windows Update on my SBS2003 box. Twice now, it has failed (hung) when doing an update to the Exchange Intelligent Message Filter. When this happens, it leaves your Exchange server in a state where it doesn't work. Isn't that great?
Anyway, I reboot the system and Exchange still doesn;t come up correctly. Turns out, I had to go to services, enable the IISAdmin service (this must have been disabled during the failed update), then I went to a cmd prompt and ran "iisreset". This seems to have solved the problem.