Do what I did.
1. Download Windows XP mode from the microsoft site
2. Using WinRAR, extract the exe file
3. rename xpm. to xpm.rar
4. extract the file with VHD in the name and the file named KEY
5. Rename key to a txt file so you can get the key
6. Rename the VHD to .vdi and it will load in VirtualBox.
When you startup, it will work a bit as it still sets up the intergration components of Windows Virtual PC. After you can log in, I suggest you install Guest additions.