Tech Source from Bohol posted a guide about installing Windows 7 on Ubuntu using VirtualBox

Do you want to install Windows 7 inside Ubuntu? If your answer is yes, then you will need virtualization software like VMWare Workstation or Sun Virtualbox to setup an operating system (guest OS) inside an operating system (host OS). The process sounds more complicated than it really is so don't worry.

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)
VirtualBox 3.1
Windows 7 Installer
1024 MB of RAM (at least)

How to Install Windows 7 on Ubuntu Using VirtualBox