I am just starting to install the Hardie board on my new house and I have about10 sqft on the bottom that is concrete. I know how to install it on concrete but is it a good idea??

You usually hang your first and lowest board so the nailing area still catches the lumber and hangs down over the seam between the concrete and rim joist.

Don't install it on the concrete. Do as ballengerb1 says. Less time, Less effort and better looking product.
Mike

Well it's on the bottom corner of side of the house. I installed treated 2x4's in the foundation to help me with this but want to now if bad things will happen(mosture behind the siding. There will be a set of stairs going up the slope so it will look good.

Maybe I am picturing this wrong but doesn't the pt 2x4 cause the siding to bow out at the bottom compared to the surface of the rest of the boards? When you mix Hardieboard and concrete there isn't much moisture can do to screw things up but try to answer my question the best that you can.

Sorry for not telling the whole story.. I put treated 2x4's in the concrete, screwed them to the inside of the forms so when you take the forms of there is wood flush with the concrete. This is normal, acoording to my carpenter.. I was just wondering if a person could get in trouble by going over concrete. It was poured last may but still has moisture to come out I'm sure.

You should be OK with what you have done. Neither concrete of Hardie plank will be compromised by a little moisture.

I agree with ballengerb1, should not have an issue. I would however, for my own take one simple step. Hardi plank, primed or pre-finished doesn't come with any finish on the back. I would throw a coal of exterior primer or paint on the back of the planks coming in direct contact with the cement walls. This will allow you to sleep better at night.
Mike

I was going to throw some tar paper in between them... Thanks