The new version of Parallels for Mac has a great new view called 'Coherence.' Windows and the Mac OS can be used as a single integrated environment with both the Windows taskbar and Apple Dock and Menubar accessible simultaneously. Additionally, the Apple Doc now contains all the programmes running within Windows as well as OSX. With a click of the mouse, the Windows or OSX desktops are brought to the front and accessible. One may easily drag and drop files between the Mac and Windows file systems as well. This software greatly increases the value, accessibility, and productivity of both the Mac OSX and M$ Windows desktops.
Another virtualisation software, VMWare, can run a virtual OS from a separate bootable partition.