Hi, i have a 2000 Chevy Blazer LT... it blows the 40 am ignition mega fuse. the starter begins to turn over without the key in the ignition, when i begin to replace the fuse but engine won't start.

It sounds like a possible grounding issue, I would check you wiring goiing to your starter, also from your main 40amp fuse box, and of coarse your battery cables.

Agreed. Something has shorted to ground, something that carries a lot of amperage in normal use, like the cables as Michael said....evidence in the starter engaging by it's self

ignition switch is shorting out and blowing the 40 amp fuse immediately when key is turned on... replaced ignition switch problem solved..shop cost is 153.00 for switch...