I'm a little surprised with the question - tenuous link to Ubuntu.
the easiest virtual-to-physical (v2p) and physical-to-virtual (p2v) tool that I use on a day-to-day basis is Symantec's Backup Exec System Recovery tool.
To P2V you would backup the machine using the tool onto an external hard-drive. Then when restoring, you would boot the virtual guest with the restore tool and reimage into the virtual guest. Most of the time, the virtual guest will boot without any changes. You can also use the "hardware changes" part of the toolset to force Windows XP to find drivers during the first reboot. Once install, install the VirtualBox Guest Additions to complete the virtual hardware device driver install.
To V2P you would do something similar - its often easier since the virtual guest has less "exotic" hardware to deal with. Once done, go through the device manager and install all the missing device drivers.
More hit-and-miss V2P solution if you dont want to buy the tool, is to go through the virtual windows XP device manager and deinstall every device you can find. Shutdown the guest and reboot it with an ISO of Clonezilla. Image off the virtual windows XP onto an external drive.
On the physical machine, boot with a CD of clonezilla and restore the image of windows XP. If you are lucky, Windows will boot with low graphics available. Sometimes you need to boot into safe mode and install a few drivers such as VGA and networking. Sometimes you need to boot the restore physical computer with the Windows XP installation disc and do a repair install.
Similarly to P2V, remove as many device drivers as possible using device manager and do a similar Clonezilla backup before restoring into a virtual guest.