I get the same problem, ever since I upgraded to Zune 4.0.
I'm running XP SP3, and have no Zune device. I have Zune software 4.0 with a Zune Pass. I'm running McAfee anti-virus software (though above, someone says that he's getting this error even with no anti-virus software running). I'm not "stressing" the system in any way.
I've tried downloading the HD version of the free preview of the "Kings" TV show multiple times. Each time, the entire file downloads (~400MB). I even monitored the download myself by executing "dir /A" commands in a command prompt window, in the "My Videos\Zune\TV\Kings\Season 00" directory as the download is performed, in order to list all files in that directory. Each time I execute the "dir /A" command, a hidden Kings wmv file is listed with a larger and larger file size. Additionally, there is eventually listed in the directory a hidden ZunCFD.tmp file, which grows to ~62MB, which I assume is a buffer file used during the download process.
I also monitor the download progress in the Zune software itself. When it hits 98% or 99%, the "Pause/Resume" download button switches to a "Play" button. Then the remaining 1% or 2% is downloaded. At this point, in my most recent attempt, the Kings wmv file is ~336MB, and the ZunCFD.tmp file is ~62MB, and it would seem that all tha needs to happen at this point is for the Zune software to combine the two files into the final Kings wmv file, and turn off the "hidden" attribute of the file. Instead, I get the "Can't Download; Your anti-virus program is slow or not responding. Error C101000B" message, and the hidden Kings wmv file and the hidden ZuneCFD.tmp file are deleted from my harddrive.
Please fix this as soon as possible. I see that this error was reported 9 days ago. I'm stunned that it hasn't been fixed. This is a major problem. I've enjoyed Zune Pass for a long time using Zune 3.0 software. If this problem isn't addressed, you are going to lose a loyal customer.
Update: I downloaded a music video just to see what would happen, and didn't encounter the problem.
Update: I found this http://support.microsoft.com/kb/967132?sd=zune , which talks of how to work around a problem in which the Zune 3.0 software invokes the installed anti-virus program to scan a downloaded file, but the anti-virus program crashes the Zune software. It provides instructions on getting Zune to bypass the anti-virus scan, or use the "built-in windows" virus scanner rather than the installed 3rd party one (I guess). I've not tried it, but it might help people get around this problem until Microsoft gets its act together on this issue and issues a real fix.