..having got XP up and running in a Virtual Machine it was bugging me to install Anti-Virus software..
out of interest I contacted a number of the major AV companies asking about licencing for running XP under Windows 7..
Bullguard and Kaspersky said that only one licence was required.
ESET and Panda said two were needed, one for 7 and one for the VM.
AVG said they didn't support Windows 7.
Symantec said it wasn't possible to install IS into a VM (??).
Macafee - well, they didn't seem to understand the question.