Which is the actual biggest complaint of developers, Vista is closing their code. The API they are releasing for it is so limited that unless you are willing to pay several thousand dollars for the advanced API you won't be writing much software in native code, goodbye all the freeware. Also they are decreasing compatibility with each new release, which is the reason why Mac took so long to finally compete with Windoze. Mac has recently released most of their API code, allowing programmers more access to the OS and increasing the amount of software available. Bottom line, if there are more programs available for an OS it will attract more users.
XP has a hard time running old DOS programs at all, and many old skool gamers still love a lot of them (there are several websites that offer them for free downloads when they can get the license or link to a place to buy them when they can't). Vista made it even worse, unless you are a programmer or know DosBox well enough you won't be able to play anything pre-XP. Windoze 7 will likely make it even worse phasing out the XP API as well.