the security light came on about 2 days ago and won't go back off.

yep,your going to have problems! sorry.Your V.A.T.S. system is screwing up.It could be the resistor in your key going bad or it could be the actual device in the steering colum.When the light begins to blink,and your car will not start because the starter and fuel pump wont work,try your other key.if it works your resistor in the key is going bad.If it still doesn't start with other key you need to install a bypass which you can find out how by searching VATS bypass. im doing mine this weekend.im buying it rather than making one,34.00 dollars

DOES YOUR KEY HAVE THE SMALL BLK CHIP ON THE GROVED PART