95 / 98 / ME were built on the same basic platform. ME failed primarily because it was released at the time when naughty boys got the taste for hacking Operating Systems...... ME was too old to build decent protection.
ME was a critical step for MS to kill off a lot of old practices. It was 98SE with certain things enforced, and some older parts disabled. Its primary purpose was to get developers and hardware makers off the old legacy methods, and start using more NT complaint APIs. ME wasn't ever planned to be secure, and Windows was being mullered long before ME. ME was a sacrifical product to kill off the 9x line.
XP was a complete overhaul and worked incredibly well, especially when Sevice Pack 2 and then 3 were released.
XP, Windows 5.1, was a fairly minor update to Windows 2000, itself a fairly minor update to NT4/3.51/3.5/3.1. Its most certainly wasn't a complete overhaul of anything.
Vista was something very new, and didn't work well at all.... to the point where some Computer Manufacturers (including the big boys) offered XP as a no cost 'downgrade' to their more savvy customers.
Vista (Windows 6.0) was a new Kernel, though not desperately different to the XP/2003 kernel. Only security was enforced, and implemented/enforced a new driver model. The 'savvy' users didn't take the downgrade option, that was left for those who couldn't get used to a new way of working, or couldn't handle the shame of not running with permenent admin rights

Windows 7 is built on the working of XP but with a totally new front end. Works like an absolute charm so far and *should* only get better with time.
Thats desperately misleading. Actually, its bordering on utter cobbler

. Windows 7 (Windows 6.1) is basically Vista, just with an extra couple of years of refinement on it. Its incorrect to claim its based on the working of XP and trying to suggest its unrelated to Vista. Its a refined Vista.
Of course, none of the above even begin to compare to Mac OS 
Of course not, Mac OS is not Windows. Personally, I prefer the open architecture of Wintel, rather than the closed, expensive, limited Mac way of doing things, although that does give the Mac some advantages
