How to turn a Windows XP partition into a VirtualBox virtual machine?
Solution 1:
VirtualBox has a wiki (including step-by-step instructions) for you:
How to migrate existing Windows installations to VirtualBox
Solution 2:
If I understand what you want to do correctly, the way to go might be disk2vhd from System Internals. It will make your Hard drive with XP into a virtual Hard drive which you can then import into a VM.