The month of December has already been unkind to Microsoft. The software giant’s Windows operating system and its Internet Explorer browser saw significant market share drops reported on back-to-back days. Not only was the November percentage drop for Windows the biggest in two years, but Windows market share dipped below a number where it has historically held tight: 90 percent. According to Web metrics company, Net Applications, Windows market share as of Dec. 1 is 89.6 percent. Meanwhile, Mac OS X posted its largest gain in two years, with 8.9 percent market share at the end of November.