Why won't Windows Excel let me have two files open at once?

I have a work computer and the Windows is still XP with the 2003 version of Excel. It does some weird things I’m not used to other versions doing. For some reason, I can’t have two separate excel files open. This is really irritating being that I have to keep minimizing one and maximizing the other. I need to be looking at data side by side. It may sound trivial, but when there’s a lot to go through it eats up time. Also, it won’t let me x out of one file without x-ing out of all the other files I have open. In other words, if I x out of one, I have to x out of every other excel file I have open at the moment. Is there a setting that can change this?