I can't tell for sure (Wikipedia has no info, and http://www.fundinguniverse.com/company-histories/Microsoft-C... only talks about their IPO), but it certainly looks like MSFT was bootstrapped when they nailed their big licensing deals. They then IPO'd after 9 years incorporated to get some more cash on hand, presumably.
Microsoft took a mezzanine round a few months before the IPO. The reason usually given is so that they'd have connections to investment bankers to manage the actual IPO, as well as people with a stake in making sure that the IPO price was as high as possible.