How do I go about using two separate OS. I know its possible and I think I have the computer knowledge to do so but I don't know HOW to do it. Also, I only have a pc. Is it possible to use two OS's on a pc? (Core 2 Duo 2GHz,120G HD, 2G RAM.)

You can set up a new partition and install the other OS on it. When your computer starts up, it should ask you which operating system you want to use and your computer will boot up the one you choose. I think this really depends on what OS you're going to install, though. I know this works with Windows 2000 and later versions but not sure about anything else.

Although some folks are going to laugh at me for this answer, you could forget the dual boot (using a boot loader) option and use two hard drives. When you want to boot from a particular OS, you make it the master drive and the other the slave. Don't laugh - I've seen it done and it works flawlessly.

It really depends on which operating system your going to install. Some have installers that can repartition your hard drives without destroying existing data and set up dual booting for you. It can sometimes fail horribly though, so it's important that you back up all important information before you even try. There's bound to be various guides on how to set up dual booting on your computer tailored to specific operating systems which are bound to be in far more detail than any answer you'll get from here.

It depends on the OS's you wish to install. If you want to install a build of Linux with Windows you need to first install Linux onto a partition of your hard-disk. You will then need to install Windows onto a separate partition.