Disconnect all peripherals from your computer.
Boot from your install disc & run Repair Disk from the utility menu. To use the Install Mac OS X disc, insert the disc, and restart your computer while holding down the C key as it starts up.
Select your language.
Once on the desktop, select Utility in the menu bar.
Select Disk Utility.
Select the disk or volume in the list of disks and volumes, and then click First Aid.
Click Repair Disk.
Restart your computer when done.
Repair permissions after you reach the desktop-http://support.apple.com/kb/HT2963 and restart your computer.
Thanks a million for your counsel. I tried as best I could to do every step beginning-to-end. But I'm no geek, either. If there was a screw-up, surely it was on my part.
Apple store guy sent me to Onyx (I already own it), told me to go to "automatic" and said it should correct the problem (what, I think, he said was an invisible ".ds store" file for desktop). I tried to follow his recipe, too, but couldn't make it work, either.
Again, many thanks for your kind counsel.