Answer:
If I understand your question correctly, you already have Windows 7 on your PC, and you also want to have Windows XP, Windows 2000, and Windows 98 running as well.
The easiest way to do this would be to install a virtual machine and operating system. I recommend VMWare Player, which is a free virtualizing system which is compatible with just about any "guest" OS. For example, I have two flavors of Linux and Windows 98 running as virtual machines on my XP host machine.
Other options would include the Sun VirtualBox, or Microsoft Virtual Server. I think VMWare Player is the easiest and most reliable.