Here are some interesting Amazon sales rank numbers as of 10/16/07 for Vista vs. XP, which Vista supposedly replaced 12 months ago. All the following are sales ranks in the Amazon software category, with a lower number implying higher sales:
Obviously this is unscientific, because it is just one channel. Also, Vista has more different segmented SKU's, so the product comparison is not exactly apples to apples. But it is interesting, no?
The OEM market is going to skew towards Vista because that is what OEM's tend to load by default. But even so, Dell, for example, is still offering Windows XP as an OEM option, a pretty unprecedented move this long after a new Windows launch. But the Amazon traffic is probably 99% OS changes, since almost everyone with a PC gets an OEM version loaded. Most of the Vista purchases are going to be upgrades from XP, and most of the XP purchases are going to be downgrades from Vista. Does this mean Vista downgrading is outstripping Vista Upgrading?
Postscript: It has now been two months since I downgraded to XP on my kid's laptop, and we are still amazed at how much better everything runs now. I was afraid I could not get all the drivers for XP but in the end I was succesful and everything, including the sound, is working great. There are a LOT of websites nowadays to help you downgrade. Or you can try dual booting.
Update: The real indicator that this is Vista downgrade sales of XP is that the Full Edition is out-selling the upgrade edition, which is a reverse of history when XP was the lead product. When I downgraded, I found I could not use the upgrade version of XP and had to use the full edition. My guess is that others have the same problem, and that the very high sales rank of the XP full edition is very likely due to high downgrade demand.