which OS

going to jump in here with personal 2-cents (or whatever small value coin)

there were some significant changes in how audio files were addressed between XP and Vista with changes carrying over to 7 . . . with regard to audio production these changes were either neutral or detrimental and a number were in response to industry DRM concerns

with windows 7, as a rough rule, it requires a lot more hardware horse power to achieve equivalent results to XP

at some point audio card drivers will not continue to support XP  at some point system critical software will no longer be supported by XP . . .

but I find nothing in Win7 that improves performance (extra memory, above 3.5 gig (more or less) is useful but not quite critical) in audio production and more then a handful of things that degrade productivity (nothing that is, yet, a deal breaker in such a way that I would refuse to use a Win7 based system . . . but stuff is moving in the wrong direction and I'm moving in direction of hardware based recording and am hoping that Linux is ready for prime time before I need to surrender XP)

for systems dedicated to audio at this moment nothing is so 'broke' in XP plus decent audio cards still available for it that needs to be fixed by a next generation of bloat from Microsoft