My favourite migration is to stick the old hard drive into a USB external enclosure, and drag the files to the new PC. The bonus is you end up with an external drive to backup the new PC.
It should be possible using a USB cable, RJ45 crossover and the software on the PC. Lets see:
- Find Windows Easy Transfer utility on the VISTA machine and Run it
- Download & Run the Windows Easy Transfer utility on the XP machine
- Plug a USB cable with crossover between them
- Create a simple network over USB
- Activate simple file sharing.
More background on what does and doesn't work, how much it costs, etc.
http://www.crncanada.ca/content/software/how-to-app-data-migration.shtml