I recently bought a DELL DIMENSION 9500.
When I check with HDD HEALTH, it warns me that the drive (western digital) temp is 60 or 70 deg Celsius. It also says CRITICAL. What can I do. Is there another way to check ?

60 deg Celsius is 140 f! That's pretty hot! Is there adequate airflow on all the grilles on the computer? Is it in a hot part of the house, say, in direct sunlight? Could there have been add-ons with your computer that may be restricting airflow across the chip?
You can get those cans of air and blow out the fans and grilles to see if that helps with airflow.