HI - I wasn't suggesting a complete re-install, I was considering a
repair install. It retains all installed programs and
most of the settings - the pain with that is having to reinstall up-to-date drivers again, and all the windows updates etc. - and a few other little tweaks. It sounds similar to what you're suggesting. It has to be run from within the current Windows system; it can't be run directly from the installation discs.
I could restore a partition backup from a week or so ago, but because Windows insists on putting Appdata on the system disc, (and won't allow it to be relocated

) a partition restore could lose me some data.
System Restore is
supposed to allow system files (only) to be recovered, but as was the case with Windows Me and Windows Vista, System restore isn't reliable - when windows breaks, System Restore usually breaks with it (as is the case now - if I try to restore to a point when I
know it was OK, the restore crashes with a BSOD - yet it will allow me to recover to more recent, but broken, restore point

).
If I can't find the root of the problem and fix it with a tweak, then my least painful option is the repair install - at least I won't have to reinstall all my programs, and I'll retain most of my user settings and user data.
The annoying thing is, I'm sure it's a registry problem to do with devices. I have Acronis non-stop backups so I could restore just the registry files back to the windows folder - if only I knew what files made up the registry.
But thanks for your input anyway - If anybody knows what files the win 7 registry comprises, I'd be gratetful for the info.