+1 to @kellenjb and @mcktimo, but let me just add: It's possible that the ground wires are there in the wall, and that someone put in two-prong outlets anyway. The fact that there are some 3-prong might mean that in fact there are proper ground wires. Or it might mean that someone bought a 3-prong outlet, then went to put it in, saw no ground wire, and just shrugged and connected the two wires. Or they might have run 3-wire cable to the kitchen but not the rest of the house.
If the ground wires are there, installing new outlets and hooking them up is a pretty easy job. It's not hard to check: turn off the power to an outlet at the fuse box (the first step before touching anything electrical, a rule I can attest to from too many times saying, "ah, I don't want to go all the way down the basement, I'll just be careful ..."), take the plate off an outlet, and examine the wires leading into it. If you have modern wiring, there will be a black wire, a white wire, and a bare wire. If it's older, you may still be able to count if there are 2 wires or three.