It doesn't return anything, or it just doesn't resolve certain details? Does it pick up other Win7 machines? Try completely turning off the Windows firewall on the machines and scanning again. If it works then, at least you'll have narrowed it down a bit more.

Doesn't return anything, like they don't exist (which is odd, because I can ping them). OK, I just tested firewall off on an xp-sp3 laptop (which I forgot about, even though it's right in front of me), and it showed up on the scan, so yeah, I guess it's windows firewall.