Help Please

is there a way i can create a macro that will do the copy and paste value
when a result cell has a value in it? ie Cell E1 has the formula
=if(D1=A1,B1,0) E2 =if(D2=A1,B1,0) what i want to do is if E1 contains a
result value stop it from changing if the value in A1 is changed - i dont
want to have to use the copy and paste value every time.

Please help this is driving me insane