Does Windows run on anything less, these days?
Despite sitting in front of Windows for at least 60 hours a week, being a Windows developer and having half a dozen PCs in the house, I have only just got my first machine faster than a P2 366.
Also, in recent years Windows has actually got faster with each version. Win2k was faster in the same hardware (assuming a sensible amount of memory is installed) as Win98/WinNT4 and WinXP is faster than Win2k.
Not many people actually need anything faster than about a P2 366. Everyone needs at least 384MB of memory though. Although if you are ripping your entire CD collection a faster CPU definitely is handy...
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Remind me to change my signature to something more interesting someday