Printing a PDF file saves .prn but won't go to printer

Windows 7 Utlimate, 32-bit.. Acrobat Pro 9.5.5. An HP Officjet and a Brother laser printer, both networked.

Whenever I try to print from Acrobat, instead of going to my chosen printer I get a Save dialog box for a .prn file. Thus, I can no longer print from Acrobat.

This started a couple of weeks ago. I have made no changes to my system, printers, or Acrobat, unless there were some automatic updates in background.

All my other software (Excel, WordPerfect, Firefox, etc.) continue to print normally.

Thanks for any suggestions.