First of all which version are you trying to reinstal and where did you download it from? There are some bogus downloads of Apo around.

You say again, so you had it installed before? are y Installing a  version in an old APO Folder or is this a fresh install. You say you have a 64 bit platform machine, I would suggest using Apo7x14 although they all should work but 7x is the latest.

Are you getting that Error when opening Apo or while running. I have had Plugin errors before when opening Apo(.dll xxx wont load/.dll invalid) seemed to always happen when Ijust added new Plugins or did some plugin swapping.

There are 100's of plugins available for Apo however you can not have all of them in the same Plugin Folder at one time. You have too many plugins in the folder at once and it will make the program unstable or it won't start. Try to keep it under 100 plugins in the active folder, also if you have just added some plugins they could be duplicates or corrupted on download. I'd check the Plugins folder first, also the name you give the Plugin folder important , Plugins3D for 3dHak or just Plugins  for v2.09 or 7x14.

Remember...No matter where you go there you are