You can borrow the Windows 7 install disk from a friend or a computer shop and install the OS using the key on the sticker that is on your computer. It is perfectly legal.

You can also make a "Windows 7 Universal disk", that will prompt you to select which version to install (Home, Pro, etc.). See http://mytechweblog.blogspot.com/2009/11/how-to-make-windows-7-universal-disk.html