Maybe we should compare each new version of the OS within their respective overall market share, so in this case, Vista has been replacing exactly 1% per month of the overall Windows market share (4.5 / 5 = 0.9% per month out of 90%) which in my opinion is quite a slow rate, given that Microsoft has already announced that they will replace Vista in 3 years time, we can project a 36% penetration of all computers by then... (or 40% of all Windows).
On the other hand, how many people will upgrade their Mac to Leopard (OS X 10.5) ? and how many Ubuntu users usually upgrade to the latest version, each released in 6-month cycles ?
This is the same of comparing IE 7 to Firefox 2.0.x which doesn't make sense as absolute numbers are affected by the huge marketing machine that Microsoft is and obviously the fact that the lion's share of Vista users are people who dumbly go to a shop and ask for "a computer"...
In the end, given the fact that more and more gamers are moving to consoles, that OpenOffice will soon become the productivity suite of choice for home users and governmental agencies and that web developers are finally writing W3C compliant code (that is, not customised for a broken industry standard), we might soon see the light at the end of the tunnel.