its not hard, just take out all the components and you will see screws holding down the motherboard, just simply take out the screws and the motherboard will come straight out.
unless you are changing your processor there should be no need for thermal paste. and as for "stand offs" which if they are what i think you mean, (i call them spacers) your old mobo should already have those underneath and your new mobo (if you chose the right form factor) will (should) fit right on top of these. to make your life simpler do not remove them.
Unless you are replacing your mobo with the exact same make and model, you'll have to reinstall Windows (as well as everything else on your hard drive).