To update you all, the PC in question only had a simple remote access trojan, which was easily removed. F-Secure Rescue disk got that. Nothing else found with Kaspersky Rescue Disk or OpenDiagnostic.
Rather Disapointingley the Microsoft Standalone System Sweeper failed to do anything, although this could be because of the dirt cheap USB stick I used to create it.
So I still don't know if it's any good.