M$ already own 90% of all PCs why should they care about quality of their products.
MacOS X is better than Windows in any aspect, Linux is great OS too buuut... Software...
Not everbody happy with Win, but if required program is windows-only, you'll stuck with windows. Of course there's Wine/Cedega/whatever and it's great but compatibility is not 100% .
MS works like drug dealer - for a first time you'll get windoze for free,either with new PC or pirated version but for work you'll need guess what... another copy of windoze. And they emm... bribe software companies to make only windows versions of software - for example games (only a few companies like id or Epic think about linux and mac users).
Vista... Too much hype about interface but they planning to steal what others already have - Vista GUI looks and feels like hybrid of KDE and Aqua. Well, they chose a good source for plagiarism, but it's MS, right ? And here comes the other side of the coin - system requirements. Vista requires more ram than Mac OS 10.4 and XP running in virtual machine on host that runs UT2004 at same time. 1GB RAM for OS itself when You need newest video card just for GUI MacOS X looks same with or without Quartz Extreme, without it interface is just a bit slower. And for Quartz Extreme you'll need 16Mb OGL 1.1 compatible video card, with 64Mb card it will be more than happy

Same for linux. for Vista default interface we'll need 256М сard w/ PS 2.0. 256M... for GUI...
Hardware manufacturers will be happy - users will need 6Gb or RAM just to run Word.