I've also used their scanner with XP and now Windows 7. In fact, I just ran it on my Vaio yesterday. It returned 17 unknown files, all of which were Sony or Vaio specific as well as my Nitro PDF Reader. That's my biggest gripe with Sophos Rootkit Scanner ... it doesn't identify a boatload of files that have been around long enough to have been entered in their database.
Having said that, the scanner seems to work because other free scanners like SuperAntispyware and Hitman Pro (that also claim to find rootkits) have never found any rootkits on my machines, either.
SUPERAntiSpyware.com | Remove Malware | Remove Spyware - AntiMalware, AntiSpyware, AntiAdware!
Hitman Pro 3 - SurfRight