need to replace PCM..... HELP!!!

2002 galant es 4 clyinder .....OK... it all started when i needed to replace the IAC....(car would start then stall) replaced with new one... (for some reason could not find one in a salvage yard around here) but anyways,... worked just fine.... now if i turn on the a/c or the heat then turn it off idle will jump up to 2500, only way to stop it is to cut car off then start back up... took it to dealership cause i couldnt figure it out.... they then said that the IAC somehow affected the PCM, and that i needed to get it replaced.. if that is so... would i be able to take one out of another galant, or would I have to get a new one..... (would rather not spend $900 on a new one) and if i can use a used one, would i be able to install it myself or would they have to do it ( i dont know if it needs to be programed or something, the net has been very limited about replacing PCM),

any help is appreciated,

be gentle..... its my first post.....