Thank you, Pondus, for your helpful response and link. I have read it through briefly and, as soon as I get some time, I'll go through the settings as suggested.
However, it seems unfortunate that, by installing an update, it should cause all these problems. It does tend to make me wonder if I have the time to deal with updates. I certainly don't have time to mess about with settings and risking the whole email system crashing at the moment. I just wish I hadn't clicked the update icon just yet.
In the meantime, I will just have to keep clicking 'OK' every time I switch on my pc until such time I can try to sort it out.
I have, in fact, got SP3 installed but thanks for pointing that out, YoKenny.
Regarding the 'ancient' Thunderbird program, DavidR, I have reverted to this after trying the 3.* versions but found the newer version difficult to get set up as I would like and not for me at the moment. The 2.* versions are still supported, I'm pleased to say so, for now, I'll be sticking with it. I'm pleased for you if you are happy with the newer 3.* version. Everyone to their own, for their own reasons, eh? Maybe when I have enough spare time, I will be in a position to try out the newer version but, reading the forums, I am not alone in reverting to this 'ancient' version.