The first thing you will want to do is get a copy of Windows 7 beta from Microsoft and VirtualBox* 2.1.0 from Sun.*I recommend VirtualBox over the others because in my experience it is the fastest, widest available across many operating systems and free.
Once you have the iso downloaded and your key, open VirtualBox.
Create a new virtual Machine. You can name it whatever you like. This will be how you differentiate several different machines if you are using more than one.
Set the OS Type to Windows/Other Windows.
You will need to set the RAM to at least 1024 MB or 1GB. You can set it higher but be wary because VirtualBox will lock this ram from your main system to be used by the virtual machine while it is open.
Create your "vdi" or virtual hard disk. This is just a file that will represent the hard drive of the virtual machine. Windows 7 needs a 20GB min hard drive.
Mount either your burned dvd or the iso file in the settings of the virtual machine. Then click "OK"
Select your new virtual machine and click "Start". It may take a moment but soon you will see an install screen similar to the Vista installer. When the option comes up, be sure to select "custom" installation. This will let you select your virtual hard drive as the installation hard drive. Afetr this it's pretty much hands free except you will also be asked for your key. During some points it may seem that it's frozen but it should be fine.
Once Installation is complete you will need to install the VirtualBox guest additions. You will need to right click the file on the "cd" and set it to Windows Vista compatability mode and run as administrator. Once this installs, restart your virtual machine.

There you have it a fully functional virtual machine running Windows 7.

I will soon be posting my first impressions of this impressive os.