For a long time Microsoft had two development paths for Windows.
There was a "simple" Windows that was basically DOS with a windowing interface. This started properly with Windows 95, then 98, then Millenium. The last release was Millenium.
There was a more complex and secure Windows that started with Windows NT, then went to Windows 2000.
When Microsoft brought out Windows XP it was based on the more complex and secure Windows 2000.
So I agree with fo3nix.
Millenium is a very un-secure operating system with no real protected multitasking.
Windows 2000, and XP (and now Vista) is a fairly secure, fully multi tasking operating system.