Crazy Electrical Problem...2003 Silverado..PLEASE HELP!

ok..so I go out to start my 2003 silverado 1500..4.8ltr..after driving about as I usually do in a day...and as soon as I turn the switch to the "on" position everything goes dead. When I turn it back to "off" everything flashes on momentarily, radio plays, lights flicker, etc. When I try and turn on the emergency flashers I get a crazy humm sound that sounds like it's coming from underneath the dash. Obviously the vehicle won't start. I've never had any problems remotely like this...and it ran great all day today...I did have to replace the battery at about 40,000 miles..but the die hard that's in it now seems to be strong.

Any Ideas?

The first thing I would do is to clean the battery posts.

ok...so it turns out I had a bad battery...changed out for a new die hard under warranty...now my truck will start up fine...but will not idle..stalls at every red light...I know a lot of newer vehicles have to go through a "learning" process to reprogram the computer...any body know what the routine is for a silverado? Or do I have another problem?