Since your PC is old, so much of dust sit on the RAM ports and shielding the contacts,
1. Clean the RAM slots with petrol
2. Re sit the New RAM
If not works, then let us know
Note: If you are using 32-bit OS, you cannot use more than 4GB of RAM regardless to any much installed
So, if you are using 32-bit os, and to check whether both RAM sticks working properly or not, then, you should try running windows with one RAM stick at a time in different slots, this will help us to find whether RAM stick is problem or Slot is Problem or everything is fine etc