People a fed up with upgrade cycles
I think that a lot of people are fed up with desktop upgrade cycles. The average 3 to 4 year old PC running Windows XP is performing quite well for all the tasks the average user needs, so why spend a lot of money (not to mention getting the compatibility headaches) for a new PC. There are no desirable features in a new desktop.
The main appeal of Notebooks or Netbooks is that they add an element of mobility, and I think the sales are driven by peoples who just want to mobilize their computing experience. A lot go for Netbooks because they are quite happy with the performance of the Atom processor.
To recover the computing industry must move beyond the traditional windows / Intel PC, which is now pretty much a comodity (even if Microsoft is still selling it's OS at premium prices). The money is in Kindles, iPhones, Android, Arm powered Netbooks etc... Equipment that offers a really different computing experience. People who already have a traditional computer will probably keep it until it breaks from now on, so that market will get stagnant.