C.6 Prevent Multiple Instances
Put the following code in your .onInit function:
System::Call 'kernel32::CreateMutex(p 0, i 0, t "myMutex") p .r1 ?e' Pop $R0 StrCmp $R0 0 +3 MessageBox MB_OK|MB_ICONEXCLAMATION "The installer is already running." Abort
'myMutex' must be replaced by a unique value!