There shouldn't be any problem installing Windows XP on Vbox. If you describe the procedure and the error message exactly, somebody can probably help you with that. There are quite a lot of howto's about on the subject as well.
If you want to perform a P2V (physical to virtual) migration from your harddisk to Vbox, then you could use vmware's free convertor program to make a vmdk file which you can either run directly under vbox or convert to a vdi file.
Here's an example howto:
Regardless of what you do you will have to renew your Microsoft authentication after the installation, just as you would have to after moving to any other hardware.
Hope that helps.