Need help with theft security system in a 97 Mustang please

I Have A 97 Mustang That I Am Wanting To Buy And There Is One Thing Wrong With It That I Dont Understand. When You Get In It And It Will Start But The Security System On It Goes Off And The Lights And Things Start Flashing Is There Someway To Get This Off Or What Can I Do. I Would Like To Buy It But The Guy Dont Know What It Is Doing He Bought It At A Auction And I Can Get Cheap. But Would Like To Get This Fixed. Any Help Would Be Great.