Detecting unistallation type on IS-MSI project

Hello everyone,
I ve got an installshield-msi project on which I put some code in the OnFirstUIBefore() function.

I wish that this code would not be executed both:
1. when doing the uninstallation through Control Panel-->Programs and features-->Uninstall
2. when clicking on the installer executable after having installed it through the same installer executable


This is part of my code which seems not work properly

function OnFirstUIBefore()
begin

if( REMOVEALLMODE) then
return 0;
endif;


...code to be avoided when uninstalling....

end


Thanx
Patrick