If you can find that gauge in the back up folder, change the extension from .gau to .gau.bak  and then try the install. If it works, change the extension back and it will be the same as before.

Usually when I encounter a problem like this on an install it works, sometimes it doesn't. I figured it can't hurt to try. It may just find something else it doesn't like and give a similar message.

Hoping for the best though.

Maybe a registry issue then? 

This is definitely a strange one.  What OS are you using Olderndirt?

The gauge problem seems to be resolved - don't ask me how  .  Downloaded my old (185.05) driver for the 8800GTS -seems fine, guess there are no registry entries for graphics cards?  The new FS9 install also seems fine - .cfg, FS files and registry entries all match date/time-wise.  The .gau file is back in place.  But the basic problem still exists.  FS9 splashscreens fine, it's when you get to the first screen and start using your mouse to select various options, that the slowdown is obvious.  Even the little display aircraft rotates more slowly, stopping when you click a selection.  The longer you try to select things the slower it gets, with blackscreen delays.  Sometimes, if you hold down longer on the mouse button, things appear to happen faster - probably wishful thinking  .  Anyway, when you get to full screen and ready to fly, shift Z reveals an initial frame rate of 000.0 climbing to 10-11 after airborne (steady 70-75 before all this).  Selecting "ALT", to change an option, basically ends the game by black-screening you into oblivion.  What little objectivity I had is gone - this needs a person who knows a whole lot more and is much more patient than me  .