To add further gloom to the scenario, after I completed the full AV scan step of the guide, Avast! reported that it was unable to scan the external HDD because "the file or directory is corrupted and unreadable". For some reason, fortunately, this PC is functioning well enough at the moment to let me browse and post. I've tried to follow the guide on those two PCs as well but I can't even get past first base, because I can never manage to complete download of TFC. I could well imagine that the rootkit is implicated in the problems this PC is having, but I can't see how it would explain the problems on the other two.
This morning, I ran rootrepeal for the first time in a while, and to my horror it diagnosed that my external Iomega HDD was infected with the MBR rootkit (I'll post the log below along with the logs requested from the guide), and so I ended up here. In parallel with my own efforts, I have spent a lot of time on the phone to my ISP (Sky), who tell me that my downstream signal is subject to very high error rates, so I have a broadband engineer coming on Saturday. Today I had the day off and have spent it all trying to diagnose and fix the problem. Browsing and file and email up- and down-load were /extremely/ slow, and then son and daughter reported that their PCs were showing the same symptoms.
Presenting symptom was poor or broken network connectivity, despite wireless network connection reporting itself to be in order. I can confirm I have been through all the steps in the malware and spyware cleaning guide full details after I describe the problem. The PC in question is a Win XP Home box, SP3 and fully patched, running Avast!, Online Armor, MBAM and TrojanHunter routinely, and Sandboxie and Secunia experimentally, connecting wirelessly to a router.