I don't know if you've noticed, but Microsoft's current score is mostly a recovery from the Vista disaster.
Microsoft's score has shot up by 8.6%, after Win 7 has taken only 11% of the market, with 63% of users still on XP, and another 15% on Vista. Let's just say Win 7 captures even 10 of that 15% Vista the next 12 month, then we are talking about another big jump in Microsoft's ratings here. We haven't even mentioned the 63% XP market share yet which will start migrating to Win 7. Win 7 is vastly, superior to XP in any way you care to mention.
Microsoft's ratings will inevitably shoot up over the next year and the year after that. In a year's time, Microsoft may very well have overtaken Apple in the ratings. After all, win 7 has only been out 8 months, and there is still 50% more Vista's then Win 7's out there.