I have Antivirus server running on Windows XP. Rather using nt-backup / xenconvert, here we choose diff approach to migrate winxp. We are using Linux to copy whole windows partition from physical machine to VM.
Followings are the steps we performed for P2V migration.
1) Install winxp on VM.
2) Boot both physical machine & VM using Fedora live cd.
3) Mount C Drive on both Physical & VM.
4) Copy all C Drive files from Physical to VM using rsync -apEogvtru, overwrite all files on VM.
5) Reboot VM, you should be able to boot XP.
Here, first step is not necessary, you may skip it & choose to recover windows boot loader as a 6th step.