Hello Adam_S and welcome to BleepingComputer!
I'll try to be as brief as possible but at the same time, hope you will understand as it can get complicated.
Fixexe.reg is a Registry File
that fixes .exe file association
that has be broken, which is usually caused by malware. When .exe file associations are broken, any associated .exe applications cannot be executed, and you will get one of those warnings/errors saying "What do you want to use to Open this file" since Windows can not recognize the file.
Hope that helps.