I have windows installed on my PC, and sometimes I would like to run it under ubuntu, is it possible?
In vmware workstation you have ability to create VM from a real system. I did that before and now this system is working in the vmware player. However, this is not a partition, but the big file with system. I don't think that this even possible, because virtualized systems can use only their own hardware components (that is virtualized) and when you'll change them in real system (when you launching windows in ubuntu) - you won't be able to login back to the real system. Because registry of the windows system will have modified drivers for hardware devices (that can be used in vm).
With VirtualBox (installable via the SoftwareCenter in Ubuntu) it is possible under Windows as well as Ubuntu to install your preferred OS into a virtual maschine.
Presently, VirtualBox runs on Windows, Linux, Macintosh, and Solaris hosts and supports a large number of guest operating systems including but not limited to Windows (NT 4.0, 2000, XP, Server 2003, Vista, Windows 7), DOS/Windows 3.x, Linux (2.4 and 2.6), Solaris and OpenSolaris, OS/2, and OpenBSD.