Windows 7 compatible hardware?
I think the comments here are missing an important aspect of the problem.
I appreciate that it has always been the case that new hardware will always run new operating systems better, but some of the systems people are complaining about aren't really old.
People (and computer companies) seem to imagine that anybody who has a system older than 2-3 years should really replace their hardware. Think what this would mean if the same were true for cars, your heating system, your cooker, or your television ( - scratch the television, the manufacturers are already managing to convince people that their 2 year old 1080i LCD televisions are not 'true HD').
What is missing is the ongoing support for the 2-5 year old Athlon XP, Pentium D, M and 4 machines that are still perfectly capable of doing the Web browsing, Email, Word Processing and home accounts, that many people still have. These are still usable machines, and the only thing that will make people dump them (literally) is either a hardware failure, withdrawal of support (as MS are threatening to do for XP) leading to their online banking complaining that their system is insecure, or a sales person persuading them that what they have is lacking in some way.
If MS wish to make XP an OS of the past, they must have an affordable upgrade path to W7, IE8, WMP 10 etc, and make sure that drivers are available for older hardware (and the same must forced on the display and audio device manufacturers who are so keen on abandoning their old-hardware customer base). After all, I'm sure that a 3GHZ Pentium 4 must be at least as fast as a 1.5GHz Single core Atom 270, which is supposed to be able to run Windows 7 without problems.
Of course, there is a vested interest in the computer manufacturers shifting new hardware, and support issues for older stuff is a useful lever to them. I've long thought that MS and the hardware manufacturers are in collusion to make sure that they continue to sell the new 'shiny' things.
Computers should be commodity tools now, not subject to the whims and fads of fashion. It sickens to see, week after week, serviceable computers and televisions being sent to the recyclers, not because they no longer work, but because the owners have been conned into thinking that the old ones are too old/slow/difficult to secure or support, and the answer is to replace them. I'm not a Green, but the blatant waste is bordering on the criminal.
A computer should be for (it's) life, not just Christmas (sic).