I think that a good reason that PCs have not sold as much as usual is that to a normal person, the PC has not gotten much faster over the last 5 years. A majority of people do only a few things on computers, including e-mail, web browsing, picture viewing, music listening, video watching, and other simple things that any 5 year old PC can easily do.
The only people that really need a new and faster PC are the ones that are into intensive gaming, video conversion, high speed audio compression, etc.
As an example, I have a Pentium 4 PC that I bought in October of 2001 with a 1.8GHz processor, 512MB of RAM, and an 80GB hard drive. Almost any average teenager or even average adult would be completely satisfied with its performance with Windows XP and have no reason to replace it, even though it is 6 YEARS OLD!
Now for me, as soon as I can convince my wife that my Pentium D 3.0GHz is slow, I am going Quad-core!