The OS so far released by the Microsoft are,

-- Windows CE(Mobile)
-- MS DOS
-- Windows 1.0
-- Windows 2.0
-- Windows 3.0
-- Windows 3.1
-- Windows NT
-- Windows 95
-- Windows NT 4.0
-- Windows 98
-- Windows 2000
-- Windows Me
-- Windows XP
-- Windows XP Media Center Edition
-- Windows 2003
-- Windows Vista

OS

The Operating System(OS) acts as the middleman between applications and hardware, with the help of of which programmers don't need to know the internals and complexity of the hardware they write codes for. Their programs just need to talk to the OS, which in turn talks to the hardware. To acheive this, the OS provides the programmers with an Application Program Interface(API), which lets them write programs that can communicate with the OS.

And if you know whats good for your computer, and security of your data you'll use something other then Windows, or Make sure you fully up-to date with all windows patches, updates, service packs, have firewall and Anti-Virus.

Otherwise, expect spyware, adware, possible hacking, pop ups and just a total system melting down in a matter of days connecting to internet.