On Windows and Mac OS X, installation requires little more than clicking on the installation file and letting it run. It's a bit more complicated on Linux and OpenSolaris. On Solaris, you need to compile the program. On Linux, you'll need to follow some additional steps, which are described in the Linux download section.
Finally, if you need more guidance, you can find step-by-step instructions for VirtualBox 2.1.0 at the Two Guys Tech site.
Setting up the VM
Your next step is to set up a new virtual machine for Windows 7. You do this by clicking the New button, which will then ask you how big a hard drive you want for the operating system. The default is to give it a 20GB virtual hard drive. With Windows 7, I decided to give it a more generous 40GB. You can also let VirtualBox dynamically determine how much hard drive room an operating system can have, but I prefer to decide for myself.
This done, you set up how much RAM and video memory Windows 7 can have. I prefer to give the operating system an ample 1GB of RAM and 128MB of video memory. You can get by with less, but you'll start noticing system delays.
VirtualBox also lets you set up 3-D graphics acceleration and access optical discs, USB devices, shared drives and so on through its main interface. You can set this up after you have Windows 7 installed, but I prefer to get this basic configuration out of the way first.
Installing Windows 7
With this done, you're ready to actually install Windows 7. You can either run the installation from a DVD or just load the Windows 7 ISO image file. Since I hate wasting time, I loaded the ISO. VirtualBox can load ISOs over both a network or from a local drive.
On both the Linux and Windows test systems, the Windows 7 installation was a snore. It took about half an hour, and the only thing I had to do was to set the proper time zone and enter the Windows 7 beta product key.
Once in place, I also loaded VirtualBox Guest Additions -- an additional set of functions that includes mouse integration (so you can mouse over from Windows 7 to your host desktop and back again without needing to hit the right-hand control button) and the ability to run the VM as a full desktop. They only work with Linux and Windows guest operating systems. But in either case, they're darn useful.