Thanks, people. Some useful hints there, and reinforcement of what I was afraid of, that all programs will have to be re-installed.
Took delivery of new HDD today, it's a Seagate, and apparently on the Seagate website there is a Disc Wizard downloadable file which makes the process fairly simple. I don't envisage any major problems there, despite not being intimately acquainted with the inside of my computer. I think my greatest fears are with losing data of various sorts, which is partly why I'm going the route of a new drive rather than just formatting the current one. At least I'll know the data is still somewhere, and I won't be formatting the old one until sufficient time has elapsed that I'm confident I've got everything I need on the new one.
On that subject, part of my disaster last time was losing my email address book. I copied the .wab file on to a floppy, ran it and checked that it worked, and felt confident. But when I tried to copy the file to my newly installed Outlook Express, no data! Just an empty address book! Why oh why does Microsoft have to make it so difficult to keep email data? The places they find to hide it are unbelievable.
Internet settings etc are the other things I worry about losing; as long as I have internet access everything else is survivable! I'll hold my nose and leap in the deep end when I get home tonight. If I disappear for a few days, you know I drowned.
P.S. Background noise, I had planned to install the new drive as the master and keep the old one there as a slave. Any particular reason why you advocate disconnecting the old one then reconnecting it as slave later?