Windows 7 does not support virtual desktops out of the box. Fortunately, there is an lightweight Desktops application by Mark Russinovich included in Sysinternals utilities.
Download Desktops application directly from the project website and install it afterwards.
Application is very lightweight and basic, so do not expect advanced features.
Mind you, every application is tied to virtual desktop it was started on and you cannot move it due to an application design in which every virtual desktop must carry out separate explorer process to provide taskbar and start menu.
Web browsers doesn’t play well with this setup (at least Firefox), but it is good enough for daily use.
Additional notes
There is also GPL licensed VirtuaWin – Multiple virtual desktops application which looks very interesting, so feel free to check it out.