Windows XP both native and in VirtualBox

There are several good tutorials about similar topics, namely:

1. How to migrate existing Windows installations to VirtualBox
2. HOWTO: Windows XP in both VM and native

but I couldn't find any that would describe what I was trying to achieve.

What I had: Windows XP installed on a physical partition. What I wanted: Being able to run the installation both inside VirtualBox and also natively. I did not want to reinstall the WinXP installation nor did I want to have to fiddle with any Windows Genuine Disadvantage stuff. And I didn't want to break the native boot in the process. Also, as this was some über-corporate install I didn't want to have to ever use the WinXP install CD + its rescue console.