This article refers to NX Client 3.5.0 or previous versions. Since NoMachine 4, the client and all the NX components are natively built on Windows (FR03F02034).

When the session fails to start on Windows and shows error 'Cannot initialize the display service',  it can be a symptom of problems related to the Cygwin compatibility layer used by the NX OSS components. NX Client up until version 3.x includes some components, such as nxwin and nxssh, which are based on Cygwin and may suffer from some known problems.

Specifically, the Cygwin Web site reports that some anti-virus, anti-spyware, and firewall applications could interfere with Cygwin. The complete list of applications which are known to be troublesome is available at:

http://cygwin.com/faq/faq-nochunks.html#faq.using.bloda

Among the problematic applications there are programs such as: Norton/McAfee/Symantec antivirus and antispyware, Windows Defender and  BitDefender.

For example, in the case of NX Client 3.4.0-5  and BitDefender Anti-virus 2010 running on Windows 7, it has been verified that the"active virus control" needs to be disabled in  BitDefender to allow the starting of the NX session. With BitDefender 2009, instead, the session seems to start without problems.

A post added to the BitDefender Support Forum on February 16, 2010 by the testing team, informs that "Cygwin1.dll problem was fixed and the update was delivered few days ago." References are available at:
http://forum.bitdefender.com/index.php?showtopic=16865&st=20&p=77037&#entry77037

As a further note, Cygwin DLL conflicts may occurr  when different programs need to coexist with different versions of Cygwin libraries. NX Client already implements a mechanism to help to solve a possible cygwin DLL conflicts. More information is available here: AR03E00449.