More information: I have an Untangle UTM (untangle.com) between me and the Internet. When I’ve been downloading AIS, the anti-virus software in Untangle, Virus Blocker (based on Clam AV), has been detecting a virus in the download file from http://files.avast.com/files/beta/6.0.1044/setup_ais.exe and cleaning the file. When I went to run the downloaded file (setup_ais.exe), the executable was corrupted and gave me an error (see attached screen shot).

Untangle Virus Blocker info:
Virus Blocker signatures were last updated: 2011-04-05 3:23:01 pm

Excerpt of log entry:
2011-04-05 12:25:31 pm blocked xxx.xxx.xxx.xxx:1212 (HTTP) http://download797.avast.com/files/beta/6.0.1044/setup_ais.exe virus found

So I’ve now turned off the virus scanner and I’m redownloading the beta. I’ll let you know what happens.