Number 4 on the list is Microsoft Windows Millennium. This might be the worst version of Windows ever released--or, at least, since the dark days of Windows 2.0. Windows Millennium Edition (aka Me, or the Mistake Edition) was Microsoft's follow-up to Windows 98 SE for home users. Shortly after Me appeared in late 2000, users reported problems installing it, getting it to run, getting it to work with other hardware or software, and getting it to stop running. Aside from that, Me worked great.
To its credit, Me introduced features later made popular by Windows XP, such as system restore. Unfortunately, it could also restore files you never wanted to see again, like viruses that you'd just deleted. Forget Y2K; this was the real millennium bug. Click here
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Credit: PC World