The default operating system selection is made by turning on the computer after the operation of the basic system I/o - BIOS. So to change the OS you need to initiate a reboot of the computer in Windows this is done from the main menu by pressing the Win. After the start of the boot cycle and appear on the list screen installed in the computer system, navigate through its rows with the arrow keys, and the choice is made by pressing Enter. This menu is displayed for 30 seconds (a timer is also present on the screen), and then, if the user did not select, download OS by default - it is the first in the list.
If when you boot up your computer this menu does not appear, most likely it is disabled in settings. In this case, use the built-in operating system mechanisms control the download Protocol. In Windows 7 to do this, first press the key combination Win + Pause, then click on "advanced system settings" and on the "Advanced" tab of the new window, click "Settings" in the section "startup and recovery".
Next open the settings window check the checkbox labelled "Display list of operating systems" and select the period to wait for user choice in seconds. After that hit the OK button, two Windows open and you can proceed to reboot to select the OS change.
Simultaneous operation of two operating systems in one computer is impossible, but there are programs that are under the control of the main OS are imitating the actions of another. If you install a "virtual machine" will be able to switch between main OS and simulated without using the procedure of restarting the computer. To find the software to implement such a scheme on the Internet - for example, it could be VMware or Connectix Virtual PC.