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.

On the browser side, Internet Explorer's market share dropped below 70 percent to 69.8 percent for the first time in more than a decade. IE slid 1.5 percentage points in November, totaling a 5.8 percent market share loss for 2008, according to Net Applications.

New challengers continue chasing IE, with the introduction of Google's Chrome browser into the market. And rival browsers Mozilla Firefox and Apple Safari saw gains in November of 0.8 percent and 0.6 percent, respectively.

Industry experts say these market share decreases for Microsoft will likely continue, so the pressure is on the software giant to compete more effectively next year with the expected releases of and Windows 7 and Internet Explorer 8.

Mac OS Keeps Gaining on Windows

A note of caution: Though the data from the Net Applications report was reported by many media outlets, Rob Enderle, Principal Analyst of consulting firm the Enderle Group, does not put a lot of stock in the report. "Since surveys like this typically have a confidence range of 3 percent, movements of a fraction of a percent aren't very meaningful."

Nevertheless, Enderle emphasized that Mac OS X has been gaining steadily on Windows for the past two years because of Vista's platform problems and Apple's strong marketing.

Enderle also has seen that Macs are sneaking into the enterprise, as more buying decisions have recently moved away from IT to the line-of-business organizations that fund the upgrades. These organizations are finding that Macs, despite their high premium price, actually provide a lower total cost for enterprises than PCs, Enderle says.

According to Enderle, overall cost in an Apple shop is less because the employees pick up much of the support burden themselves rather than relying on an overburdened IT department. Often, the employees are even encouraged to buy their own Mac machines, dropping the annual cost below $1,000 per employee per year, he says.