Before we start:
Make sure the costumer believes he is stupid, needs to trust whoever he pays for his software and services, any computer-problem is his own fault and needs to buy a new computer every 3 years.
01. Make sure the CD or DVD has to be paid for.
02. Make sure that before install a huge EULA is being presented.
03. Make sure the CD cover has a serial number.
04. Make sure that number is to be inserted somehow somewhere during install.
05. Make sure the install takes a loooong time.
06. Make sure a fictional help-desk phone number is available.
07. Make sure the CD/DVD is not allowed to be redistributed.
08. Make sure to let the kernel crash at least once a day.
09. Make sure to let the costumer know he needs to install (and buy) a firewall-client, a virus-scanner and spyware-protection.
10. Make sure the Linux OS has to be reinstalled every few months or so for whatever reason.