Often no-one posts a solution to what they did, they just say it's fixed, which is unhelpful.

I looked up how to disable SpyBot's Teatimer, found a web page which detailed it, recalled doing the exact same thing last year.

Disabled it, restarted, ran the installer with no problems, enabled teatimer, restarted and ran the scan - came back all clear.


Thank you for your help Lord of the Rings, took a few tries to get back to your original suggestion.

The history for Teatimer showed that it deleted each encounter of Win32.Agent.oc, regardless of whether it notified or not. Without that, the installer became "corrupted". But it's not just SpyBot that detects it as a trojan.