Rogue Leader :
If you extend the screen you can maximize different applications in windows and do 2 totally different things yes.
If you're asking to have a different OS or instance of Windows running no you can't do that.
Not entirely true.
You could have virtual machines running on a different monitor, could use remote desktop, or could have2 separate PCs and link them together with synergy program so you can use the one keyboard and mouse.
But as stated yes you can run different applications on each monitor, like have excel open on one screen and the internet on the other. Dual or even triple monitors is a must have for web or database programmers.