if the two files are the same size and resolution you can choose "duplicate layer" and choose the other file (both files need to be open) as the target. it should place the layer in the exact same position as the source file.

 

another trick is to change the colour mode of the placed layer to "difference" and move it around until it is mostly (if no changes, then completely) BLACK. this means all the pixels are aligned. you can then switch to whatever colour mode you want. (remember to "upper" layer needs to be set to Difference or you won't see the effect)

 

hope this helps

scott