Old hard drive new computer

Question

I want to install a hard drive from my old computer into my new computer as a slave drive. The old drive is a Western digial 15.3 GB with a 39 pin connector plus a 4 pin power connector.
The new hard drive in the new computer has a much small pin connecter, plus the same looking 4 pin power conencter.
The new computer is a HP Pavilion a6312p PC.