P.S. UPDATE:
1. I have now managed to run MalwareBytes Anti-Malware. It took a fair (c. 5 hours?) while to run (including the ext HD) but here are the results:


Malwarebytes Anti-Malware 1.75.0.1300
www.malwarebytes.org

Database version: v2013.08.08.04

Windows XP Service Pack 3 x86 NTFS
Internet Explorer 8.0.6001.18702
Xxxx :: Xxxx08 [administrator]

08/Aug/2013 13:26:33
mbam-log-2013-08-08 (13-26-33).txt

Scan type: Full scan (C:\|E:\|)
Scan options enabled: Memory | Startup | Registry | File System | Heuristics/Extra | Heuristics/Shuriken | PUP | PUM
Scan options disabled: P2P
Objects scanned: 855412
Time elapsed: 4 hour(s), 58 minute(s), 4 second(s)

Memory Processes Detected: 0
(No malicious items detected)

Memory Modules Detected: 0
(No malicious items detected)

Registry Keys Detected: 0
(No malicious items detected)

Registry Values Detected: 0
(No malicious items detected)

Registry Data Items Detected: 0
(No malicious items detected)

Folders Detected: 0
(No malicious items detected)

Files Detected: 0
(No malicious items detected)

(end)


2.
After doing the above scan the whole PC seemed to become v sluggish even with nothing open. Nonethless I tried to Install TDSSKILLER. This rather worrying screen then appeared. So after about 10 minutes I got worried and killed the window and rebooted the (WinXP) PC. I had a look in the Control Panel to check that nothing had been installed... And then I tried running TDSSKILLER again. But again the same rather worrying looking screen has appeared. I should explain that withing the window there is a flashing cursor pinging around at very high speed randomly all over the blank area of the window. From within Windows Task Manager I can see that that the CPU usage is bouncing around at about 50-60% so it definitely is doing *something*... Should I just let it run or what? And if so how long should I give it before killing it - esp given that I still have the external HD attached?

Screenshot here:
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