I'm also going to toss in a few points:
Keep in mind a lot of operation systems are no longer the same operating system as they were when they were first originally installed or released
System compatability updates, system upgrades and driver changes can all attribute to various performance hits/gains.
The largest noticable example of these changes is the jump between Windows XP SP2 and SP3.
While running SP2 a computer could run smoothly (minus any background running processes or the like) with only 512MB or RAM. Once you install SP3, the smoothness is no longer as prevalent unless you've got at LEAST 1GB of RAM (in most cases requiring more depending on the software and different application packages you have in the background).
Also don't forget the software being used on the system is also a major factor, as well as the overall increase of software usage as a user gets more technically inclined.
Example: Office XP was a much lighter and less resource intensive version than the new Office 2010 (which now has a supposed minimum requirement of 256MB, but I can damn well tell you if someone expects to be able to do more than JUST open a spreadsheet it's a pipedream).
It used to be a new user would only need a web browser and maybe notepad, but i've discovered people who consider themselves "computer illiterate" and are unknowingly using multiple windows, office products, quickbooks (and lord does the new 2010 and 2011 version require a monster of a machine in most cases), and video players simultaniously and complain about how their "old computers" can't keep up (they just need to stop opening 20 things at once while only using 1 thing in most cases).
very thought provoking question!