i did this once when mounting something with powerISO, it forgeted everything i had when i selected the wrong thing. not sure why.
start - run, type "command"
in the prompt, enter "cd\windows"
it should do-hicky the windows folder for ya
now enter "regedit"
find HKEY_CLASSES_ROOT\.exe
change it's default value to exe or exefile or something like that
then find HKEY_CLASSES_ROOT\exefile\shell\open\command
change values to "%1" %*
close regedit, restart comp, and cross your fingers