Can anyone tell me how to install two operating system on one PC such that both operating systems can be used at the same time?
You can use a virtual machine to do this, and allow it to operate in seamless mode.
VirtualBox is a nice cross-platform, free application which allows you to do this. You can emulate nearly every operating system under one-another. The full list of compatible guest operating systems can be found here.
The following is Mac OS X emulating Windows XP in seamless mode:
And the following is Mac OS X again, but emulating Mandriva Linux:
While you can use seamless mode, I find it much easier to just switch between them using Fullscreen mode... Although this depends largely on the purpose of the virtual machine.
You cannot use use two Operating systems at the exact same time without some virtual environment (e.g. a hypervisor).
You can install two and use them independently of each other, however. For example, you can do a clean install of Windows XP and partition your hard drive into two partitions during the install. The Install Windows Vista or Windows 7 or some flavor of Linux to the second partition. This will allow you to choose which OS to start during each boot.