Computer problems

Hey, my friends computer for some reason when ever he tries to run a program it comes up with a window asking what program to run the program with, if that makes sense, anyways I know there should be a way to fix it I thought it might be in folder options but I could not find anything so I was hoping someone would have a idea on how to fix this? Also he is apparently missing a file called “rundll32.exe” so a bunch of programs can’t run without this file, can anyone maybe post up a copy of thiers for me to put on his computer? It should be in “x:WINDOWS/system32/” Thanks.