Windows 4 All

Windows4all is a Microsoft Silverlight based online virtual operating system. It is a website that emulates an operating system inside a web browser.

Windows4all.com replicates the desktop environment of a modern operating system. The user can launch applications right away without installation. -Wikipedia

Besides desktop, taskbar, start menu and sidebar, currently it includes such applications:

• File Explorer
• Internet Explorer
• Video Player
• Rich text editor
• RSS reader
• Chat
• Notepad
• Paint
• Calculator
• Games like Chess, Solitaire, Spider etc.

Have a look at Windows4All

Tip: While using it press F11 and your browser goes in full screen mode and you will get the feel like you are running an actual OS.