I found the problem solved after doing the following:
* open windows start menu
* type cmd and wait for cmd.exe to appear listed under the title Programs:
* right click on cmd.exe and choose "Run as administrator"
* a command prompt opens, then type the following: SFC.EXE /SCANNOW
* hit enter and wait for Windows to check and repair its system files
Note: although Windows reported no violations were found in my system, I've found that Explorer now loads thumbnails almost instantly