I use several computers one with Vista Business, one with XP Pro and one with XP Home.
A lot of the times they will all be running.
There is definite a slowing down process with Vista, I am sure the reason being that Vista is really fierce on the security issues and examines everything minutely before letting you in.
It is not just your website I have experienced this but with virtually all websites.
It is interesting to note that my latest computer, a fairly high spec one, from Dell which I specified XP Pro instead of Vista (you have to pay extra for this, believe it or not) the response time for anything on AC is instant.
Also are the guys out there aware that their ISP's have them sharing the line with up to 100 other users? Can you imagine the traffic at, say, between 6-11 p.m