Why have all Windows .exe files changed association?
Unless the registry was modified, Microsoft Windows does not allow for the exe file extensions to be changed. Therefore, it is likely that one of the two scenarios below has occurred.
Computer is infected with a virus
The computer may have been infected with a virus or other malware. Viruses commonly attack executable files and may have infected each of the exe files on the computer making it appear that the association was changed. Make sure you have an up-to-date version of your antivirus scanner and scan your computer for any viruses.
Software utility
Utilities such as virus scanners may change the way Windows handles executable files to help protect your computer from viruses. If you have recently installed a virus scanner, file checker, or other utility and your computer is now experiencing this issue, we recommend you first uninstall that program.
If uninstalling the program does not resolve your issue or you have not recently installed a virus scanner or another utility, we recommend you restore a backup of the registry. This action may be completed using the Windows System Restore feature.
If this also does not resolve your issue or you do not have a backup of the registry, we recommend you reinstall Microsoft Windows.