Users of Avast! and Norton have recently reported issues either with installation or downloading. This is because of the checks that both applications carry out when installing a new software, not because they have found a virus!

Avast
Starting from version 7 Avast has introduced a new control mechanism to detect potentially malicious files. The AutoSandbox feature will attempt to block NoMachine binaries because it considers that "The file prevalence/reputation is low".

In this case, if you want to keep AutoSandbox turned on,  it is necessary to create a rule for each NoMachine file to allow the session to start.

For more information about how to add the rule or disable AutoSandbox you can refer to the Avast Support Center:

https://support.avast.com

https://blog.avast.com/tag/sandbox-en/

We are aware that Avast tries to base its rules for reputation level on the popularity of a particular application amongst its users. The low usage of NoMachine software amongst Avast users will result in a low reputation warning.

Symantec
Norton blocks the downloading of the software package with the following report: WS.Reputation.1 .

This is not a virus, but a generic threat description created by Symantec to classify the files that have a low reputation in their  reputation system. We don't know all the specifics behind their rules,  so we are unable to provide a detailed explanation of why NoMachine software produces this warning.

What we do know is that one of the rules is how "new" a software is and this is a criteria of the Preview packages in general: we release them every 60 days. We can confirm that our software does not pose a security threat to your system: it is not malware nor does it contain viruses.

Anyway, these are all the links explaining the topic:

http://securityresponse.symantec.com/security_response/writeup.jsp?docid=2010-051308-1854-99
http://community.norton.com/t5/Norton-Internet-Security-Norton/Clarification-on-WS-Reputation-1-detection/td-p/232155

In the meantime, we are submitting our files to the whitelist program of Symantec Norton.