1997 Volvo 850 97 Volvo 850 Won't Start

1997 Volvo 850 6 cyl Automatic 135K miles

I just bought a pre-owned 1997 Volvo 850 (non turbo). I have only had it for one week and the dealer isn't helping me one bit. He does not care. So, here I am.

The car drove great during the test drive of course and for 4 days after I bought it. 2 days ago I stopped at a gas station on my way home from work. Nothing about the car seemed odd while I was driving that day or when I shut it off. However, when I went to get back in the car, it would not start. It felt like it wanted to but would not crank over. The dashboard warning lights lit for a split second then faded out. I got a jump start, which started the car right up. I drove around for awhile then went home and parked the car. Little while later, same problem only this time, not even the slightest hint of wanting to start. NOTHING happened at all. I got a ride to the auto parts place and bought a battery. Connected the battery and still nothing, but the hazards came on and stayed on until the battery was again disconnected. Yesterday, I had it towed to a mechanic. After work, I go to the shop and the mechanic tells me that when I switched the battery, it triggered an anti theft mechanism and it just had to be reset. He showed me how to reset it should this happen again. I got in my car and happily drove home. Mind you, I still have to get the radio code as that is needing to be reset now as well. In any case, I go to get in my car this morning and guess what? No sign of life whatsoever in the car again. NOTHING happens, same as before. I tried the little trick the mechanic told me about. Nope, nothing. That mechanic is not open on weekends of course. So, I called a dealer here in town and they said what the mechanic told me was impossible because Volvo did not have such built in anti theft mechanisms until the 98-99 Volvos came onto the market. If that is so, how did the mechanic come to that conclusion and get the car to start by resetting an anti theft mechanism? Also, if this is the case, why do I now need a code for the radio? Well, the dealer told me that was possible for the radio issue to happen when disabling a battery but nothing such as an anti theft mechanism would cause the car not to even crank. The dealer I called said I may have a bad starter. Could this indeed be the case? Will a bad starter cause the car to seem dead as a door nail and not even crank over? I am not car savvy and I need some serious help! I don't have tons of $$$ to get this thing fixed. Seems like once its running, it runs great so I am hoping this is something relatively simple So, any advice?