This turns out to be pretty difficult… but here is a shot:
Computer Name
Username
Group
<?relevance
trs of (td of item 0 of it & td of item 1 of it & td of item 2 of it) of (name of item 0 of it, values of result (item 0 of it, bes property "user name"), item 1 of it ) of (members whose (exists relevant fixlet whose (name of it as lowercase contains "windows xp service pack 4") of it) of it, names of it) of bes computer groups
?>
I think this will have performance issues if you lots of computers, but I would guess it works for most deployments under 10,000 agents… Let me know if it works for you…