I'm not saying this is it, 3dobermans, but Avast, my antivirus program, has been pestering me Very Annoyingly for the last week to buy another year's subscription, popping up in the middle of a game and tossing it to the task bar. They pestered me so badly I had already decided to tell them to take a hike. I was about to when I got a little note from Malwarebytes about updating. I couldn't. I couldn't do a thing with it and 'the server could not be found, please report the error' but there was no error number to report.
Okay, I uninstalled Avast early. Not only was I able to get Malwarebytes updated but my email program started working again and I'd been trying all day to figure out why no emails were coming in and I was getting an error report about Live Mail.
As far as I'm concerned that was not a coincidence. I downloaded MS Security Essentials and that is darn well that.
Er...in short, could it be your antivirus program stopping the update?
"Best not to think about it. I don't want to fall to bits 'cos of excess existential thought."