It is a very technical case, However, if you have know how of the technical side of computers, it should not be difficult for you. First of all install Windows XP or your previous operating system on one partition. Properly install and run it. Then start operating it and while you are on the XP desktop, insert the DVD for windows Vista or anyother OS that you want to install and run setup. While installing select the other partition to install it into. The system will automatically create a dual boot menu for you when you start it. See the link below for details:
Multiple OS
Multiple OS
