On my system with onboard RealTek I only get the option for virtual surround if 2.1 speaker system is selected. Have you got a drop down for 5.1?
Do you have Realtek Audio Manager in you system tray? If so right click on it and see if audio devices is available (if not you will have to access this through the control panel). On playback highlight speakers then click on configure and ensure that you have 5.1 selected and then test. You will also be able to test the windows sounds through the sounds tab.