Name "temp"
OutFile "temp.exe"

Section ""
  Call checkForUninstall
SectionEnd

Function checkForUninstall
  ReadRegStr $R0 HKLM "SOFTWARE\legacy" "name"
  IfErrors done
  MessageBox MB_OK|MB_ICONINFORMATION "$R0 inside"
  Abort
  done:
  MessageBox MB_OK|MB_ICONINFORMATION "$R0 pass"
FunctionEnd