97 volks. Jetta - Battery - "drain in system"

I have a 1997 Volkswagon Jetta. I bought a battery for the car over 3 years ago. Last weekend my car was dead and I had to jump the car to make it start. Later that night, it was dead again. The next day I jumped it again and brought it to Sears repair and they tested the battery. They said that there was nothing wrong with the battery. So 5 days later, it happens again. I had to jump the car and bring it back to Sears. They said that there was "drain in system" and referred me to a specialist. My parents told me to buy a new battery and see what happens. Please help!