I can see this working if you do it in three steps.

1 - Use a compression utility like WinZip to make an archive of all the JPGs you want to retain. Since they all have the same folder name, you'll need to retain the folder structure, as well.
If you track down a copy of PKZIP (the old DOS-bases ZIP utility), you can do this from a command line. WinZip has a command-line tool, as well that works wonders.

2 - Delete ALL the JPG files using DEL /S *.JPG

3 - Unzip the archive you made to restore all the files named FOLDER.JPG.

Tah-dah!


If you go the PKZIP or WinZip CLI route, you can make a batch file that will perform this operation automagically.