A little history of Microsoft…
Windows 3.0 - the first windows where people actually started using it, ran in 286 protected mode.
Windows 3.1 - minor changes to the above with some 386 enhancements, but still basically ran in 286 protected mode
Windows 95 (which identifies itself as Windows 4.0 - things make a lot more sense when you look at version numbners) - compatible with older versions of windows and DOS. Minimum requirements are based on what most people have available, and the thing will barely run on the minimum requirements (486 with 8 MB of ram, IIRC). Microsoft takes a lot of flak for setting the minimum requirements too low, but also takes a lot of flak for making an OS that won’t run on a lot of hardware. You can’t win.
NT 4.0 - the “business” version of windows. Has a better kernal, better protection, but a lof of the things that make it “better” also completely break compatibility. Also requires a higher horsepower CPU to do the same thing, since programs can’t directly access hardware to speed things up.
Note at this point that Microsoft has two completely different OS lines that they are maintaining, “Windows” and “NT”.
Windows 98 (which identifies itself as Windows 4.1) - As the version number indicates, it’s really just a minor improvement to 95. The minimum requirements are pretty much the same as for 95, but are listed higher becuase of all of the problems that Microsoft had with the minimum requirements for 95
Windows 2000 (which identifies itself as NT 5.0) - At this point Microsoft doesn’t want to maintain two different OS lines. They want to kill off the “windows” line because it has a reputation for being unstable. Note that a lot of this instability comes from being backwards compatible to DOS. Microsoft says they are “merging” the operating system lines. Unfortunately, most games and a lot of home software don’t run worth a crap on 2000. Halfway through development, Microsoft completely shifts gears. 2000 is going to be for business folks only. Minimum requirements are much higher than NT 4.0 because of significant changes to the OS, prettier graphics, and a lot of multimedia apps that are thrown in because 2000 was supposed to be for home use also. Ignore the multimedia crap and 2000 runs just fine on a much smaller machine, although it is much more of a memory hog than NT 4.
Windows ME (which identifies itself as Windows 4.9) - Microsoft hypes this up as the “home” version of windows 2000, but if you pay attention to the version numbners you realize that it’s only a minor enhancement to windows 95 and 98. Microsoft didn’t have a “home” version of 2000 (since they intended only to have one version of 2000) so they threw this OS out the door in a real hurry. The took a lot of the features of 2000 and hastily ported it to 98, called the thing ME, and shoved it out the door. As a result, it’s a buggy piece of crap. They intentionally crippled things like DOS mode just to get you used to not having DOS, because they intend to force all home users to NT whether they like it or not. Even though it looks a lot like a new OS, it’s just 98 under the hood. The increased minumum requirement come from the multimedia stuff that is bundled with it. The OS itself takes up pretty much the same resources as win 98.
Windows XP (which identifies itself as NT 5.1) - This is also hyped as a completely new OS, but if you pay attention to the version numbers, you realize that under the hood it’s basically the same as windows 2000. “Windows” is officially dead. We’re all running NT from here on out. XP is basically 2000 with a huge facelift, and its all of these pretty graphics that result in the increased minimum requirements. XP and 2000 will both run quite happily on 128 MB of RAM, but microsoft bumps up the min requirements on both to insure that things run smoothly no matter how many windows the user has open. If you turn off the “eye candy” then XP will run on just as small of a machine as 2000.
Microsoft also keeps the minimum specs pretty high because they don’t want to support old hardware. The less stuff they have to support, the easier it is for them. If you have old stuff, you’re screwed. Microsoft’s advice? Buy new stuff. Not only does Bill get more money, but so does everyone else in the industry.
Microsoft seems to be foundering a bit with Vista. They are playing catch up to Apple, so a lot of the higher system requirements are a result of new multimedia apps. Microsoft is also switching to a graphics system that looks prettier, but requires a lot more computing horsepower. Vista is going to be a significant change “under the hood”, so it’s basically like the switch from NT 4 to 2000. A LOT of things are not going to be backwards compatible. This is a notable difference compared to the switch from 2000 to XP, which was basically a facelift with a few new features thrown in for good measure. One of the key features of Vista was supposed to be a much more secure file system. Part way through development, this was completely scrapped, so I’m guessing they found some major security hole that made the whole thing impractical. Microsoft has been rushing around ever since trying to find enough things to stuff into vista to justify coming out with a new OS. Vista’s code name during development was “longhorn”, and a lot of folks were calling it “shorthorn” due to its lack of new features. I personally haven’t had a chance to play with vista yet. I’m a little underwhelmed by what I’ve heard about it so far. We’ll see what it’s like when it comes out.