I have some more information. Not sure if this is helpful, but…

I have just re-scanned previous versions of this file (I keep them for reference just in case). All versions are now showing the same infection with the current virus definitions, however no problems were detected when the file was originally downloaded. The most recent version was downloaded on 4th Jan 2011, so I suspect something has changed in the detection signatures (or whatever is used by Avast) since then.

Avast Auto-update is enabled so I should have been using the “latest and greatest” virus definitions when these files were downloaded.

Thanks for all the support received so far.