Hi stickby, this is for Win 2000, but other Windows versions are similar.
When you're working in regedit.exe (loaded from Start>Run>Regedit>OK) Click on "File" and click on "Import". The resulting dialogue box allows you to navigate to files of the type *.reg. Any files of this nature are those you have created when you "Export"ed the Registry and gave it a name. You can export the whole registry or just a single key and choose an easily remembered name.
The Registry itself consists of many files in two key locations. They are:
C: \WINNT\Repair
C: \WINNT\System32\Config
You can't muck about with them directly.
If you're working in the more powerful regedt32.exe, the routine is different. Click on "Registry" and then on "Restore". The same priciples apply as for regedit.
My Saga has improved but my question remains. What's the point?
It turned out that during my rebuild IE6 had not loaded properly and had unplugged itself (leaving OE6!). Windows Explorer couldn't load properly either. So an uninstall/reinstall of IE6 fixed that and another registry "Export" was done. An immediate "Import was satisfactory with errors caused by keys in use. (To be expected).
But given that Windows was damaged and, could not be fixed by a Registry Import, I remain doubtful that backing up the Registry will always be useful.