Microsoft has never sold anti-virus software - aside from Windows Security Essentials, of which Windows Defender which is built-in - and both are including in your purchase of windows. This stuff didn't come around until 2009 however and so if he was creating viruses to create a market, he wasn't taking advantage of it for a decade? Nah.

On the other hand:

It is hearsay that John McAfee, founder of the McAfee antivirus software company, spread a virus to demonstrate the capabilities of their product in the early days of the company which must have been the mid eighties.

Edit: Just saw this article from 2005 where gates talks about entering the anti-virus sector/anti-malware security