I found the above error on a Windows XP machine, when trying to update the virus definitions.I checked the Internet Explorer proxy settings - automatically determine was not checked, and there was no proxy setting.
I also ran proxycfg -d, just to make sure.
This still didn't fix it.
In the end, I ran wireshark during the update attempt. I discovered that, despite the proxy settings being turned off, MSE was still querying the dns for host name wpad, and downloading the proxy setting script from http://wpad/wpad.dat.
Which would have been fine, except that I had a stray CNAME record for wpad in my local DNS, that pointed to my local webserver, that had a stray wpad.dat file in it, which set the proxy to a machine that no longer exists!
What was worse, the stray entry only existed in one of the two DNS servers on my network, so the problem was intermittent, depending on which DNS server Windows used for the query.
Once I deleted the stray DNS entry, and removed the wpad.dat file from my web server, it all started working.
The moral of the story is, if you experience the same problem, try typing http://wpad/wpad.dat into your web browser, and see if you get a file. If you do, open it with a text editor, and it will show you what proxy settings MSE will decide to use (even if you have told it not to)!