Half of PCs won't run Vista
http://www.eweek.com/article2/0,1895,2068351,00.asp
I can believe it. I've been running Vista on a system that is about 2.5 years old. Its not horribly slow, but its not lightening fast either. I tend to buy highend systems when I plunk my money down, but I can imaging lots of issues with people that don't follow that paradigm.