This is a simmple, compact calculator written in batch. It has a screen with instructions, and a super small window, making it easy to keep out of the way. It also shows your last answer at the top, and has a few extra functions. Download it here.
The reason this is a .bat file and not an executable it because it keeps it to a slender 2 kilobytes, and unlike other projects of mine, I don't care if it is modified and distrubuted.