OK then chaps...
...the response from the guru (who is a MS technician) is more or less along the lines of mixture's solution. He raised the point of the latest upgrades to Avast! "appearing" to cause some probs with Vista. So, he's going to break into the system, get it running and leave avast in to see if the prob recurs. If it does, remove Avast and install MS security essentials. Then give the complete system a jolly good "fettle" (aircrew jargon).
My problem was such that all the solutions that suggested download this download that, were non-starters, because the only page I could access was the page saying "you've got a problem" No bottom tray meant control panel was equally inaccessible and at that point my knowledge ran out. Which is why I'm happy to pay an old mate to do that bit for me (even then his arm needed a deal of twisting to take it).
Thankyou all. I'll leave this up, as prevously suggested by SD, so that others may be able to pick something useful from it.
The Ancient Mariner