Is it my alernator or battery? Or is my starter getting old?

I have a 1999 ford contour, 4cyl with 76856 mi. On odometer. Here is the issue I have been having. I pulled up to a gas station one night and left the lights on while I went into the store for less than 3 minutes, and the car was dead and wouldn't start when I came back out. I tried to start it but it was trying real hard to turn over but it seemed the longer I held the key into start the less power the car had. I got a jump and the car was fine, left it run for 30 min and later it started right up. However, recently just starting the car seems to drain the battery. It it sounds like it wants to turn over, but the juice just drains as I am attempting to start it. It so bad that even after a jump and 45 min of running, it won't even turn back over the next time I start it. Whats up with my car!