02 Suburban won't start - security?

When I open the door, I hear 3 dings, the dash lights flash, and the parking lights flash. The battery was replaced less than 5 months ago, and the cables are tight (now) battery is fully charged, but it won't start. Anybody got a clue? :banghead: HELP! ...my other GM is in the shop for some other electrical ghost - I'm ready to buy an import! :disappoin