ask to install message box?

[deguix]

You can mix both of them to create what you're looking for:

PHP Code:

Function .onVerifyInstDir
  IfFileExists "$INSTDIR\\Winamp.exe" PathGood
    MessageBox MB_YESNO "The destination folder is not a Winamp folder. Do you want to continue?" IDYES PathGood
    Abort
  PathGood:
FunctionEnd