The difference is caused by the absolute positioning of your paypal image. This will remain where it is regardless of the screen size, which means it's in the wrong place at some screen resolutions.
You might well be better reworking your layout to use a simple two column layout - there's a good example here
. This might seem a lot of work (it isn't..) but will give you a much more flexible layout which will be simpler to amend moving forward.