There is some evidence to support the idea that I have an infection. MBAM needed to be renamed to run and then found (quickscan) what it identified as two Trojans HKEY_CLASSES_ROOT\t1w.ourmimefilter
I let it remove them.
I ran Hitman Pro but the results were inconclusive - a suspicious file was found but it may have been OK. I deleted it anyway.
I am currently also running SuperAntiSpyware.
I will also run ZA as you suggest but it will be only zaSuiteSetup_91_008_000_en.exe since I cannot install zaSuiteSetup_91_507_000_en.exe either.
This hasn't yet been enough to resolve the problem. More results later.
None of the Malware checking produced any convincing results. I might have missed something but everything these programs found seemed OK. In one case a file in one directory was marked as a Trojan but the same file in another wasn't. ZoneAlarm's scan found nothing amiss.
I suspect I'll have to try rebuilding Windows. more later.
Tech Support has sent this issue "up". More later.