I've had a lot of students and teachers come to me with this question lately. An entire generation of computers that were cheap but decent a couple of years ago (think Celerons and P4s with a quarter to a half gig of RAM) have begun to crawl under Windows XP in the last few months.
This type of computer also represents the mainstream for many of our users' desktops, and, chances are, they're coming to you with their own set of performance woes.
While some of this can be cured by running anti-malware software (I still swear by Lavasoft's Ad-Aware; Microsoft's Windows Defender is actually pretty good, too), much of the problem relates to the security patches, antivirus, and firewall utilities that most of us run to keep truly crippling malware at bay. By the time Service Pack 2, Norton Antivirus (or AVG, or McAfee, or whatever), Google and Yahoo toolbars, and a host of other patches are running, there just isn't much horsepower left in a single-core system for actually running applications.
This leads to one of three conclusions. The first is that everyone should upgrade to dual core (at least) machines with 2 GB of RAM. Yeah, right. While this is obviously not going to happen for most of our schools or our users, it does point to the fact that ongoing upgrades are a must. 8-year old computers are not OK for our kids or our users.
The second possible conclusion is that we somehow need to look beyond the traditional desktop upgrade model: buy a new computer, wait for it to become (or at least seem, relative to other newer computers) intolerably slow, and then buy a new one. This is where thin clients can come in very handy, radically extending the utility of desktop access devices and allowing for upgrades to primarily happen as needed at the server level. For both Windows and Linux thin clients, performance is certainly enhanced by high-powered processors, but is largely dependent on RAM, a cheap and easy upgrade.
Finally, one could conclude that we need to find an operating system that runs relatively well without too many anti-malware utilities. Of course, this OS already exists in Linux. In fact, I really believe that most of the performance gains that Linux devotees attribute to Linux are much more an artifact of less overhead from utilities to keep them clean.
Fresh XP installs are actually pretty snappy, even on older or slower hardware. It doesn't take much time living with Norton Antivirus to wonder where your performance went if you don't have a dual-core to run the AV functions in the background.
Whether it's because the install base is too small for hackers to bother writing "Linux malware" or because Linux is inherently more secure than XP, most flavors of Linux work quite nicely on single-core hardware without intrusive anti-malware utilities. Even if this only gives the illusion of speed, the users, students, and parents I've converted have certainly managed to extend the life of computers they would have otherwise tried to replace.