If you are ready, here's a step-by-step instruction on how to install Windows 7 (guest) inside Ubuntu (host). But first, some requirements:
Ubuntu (9.10 preferred)
Windows 7 Installer
1024 MB of RAM (at least)
Now let's get started:
1. Download the latest version of VirtualBox from HERE, and then install it.
2. Navigate to Applications --> System Tools and open VirtualBox.
3. Create a new virtual machine by clicking on the "New" button to start the setup wizard.
4. Create the name of your virtual machine (VM), in this case "Windows7", and then select the Operating System and its version (see below).
5. Specify the amount of memory (RAM) to be allocated to the VM. I've got 2048 MB of RAM so I decided to use half of it to make Windows 7 run a bit snappier.
6. Now create a new virtual hard disk and just click on "Next" to start the Create New Virtual Disk Wizard.
7. For the disk storage type, I use “Dynamically expanding storage” as I always do.
8. For the virtual disk storage size, make sure to allocate more than 5 GB for you to successfully install Windows 7. Click on "Next" and finish it up.
9. Windows 7 now appears on my list of virtual machines. Double-clicking Windows 7 will open the First Run Wizard. Click on "Next".
10. You will have to select the installation media for the guest OS so make sure the Windows 7 DVD installer is loaded.
11. Finish it up, and then a prompt will appear (see below). Just ignore it and click "OK".
12. Windows 7 installer will now be running in a moment. Just install Windows 7 like you normally would, and see the finished product :-)
Here are a couple of screenshots of Windows 7 running inside my Ubuntu desktop: