How to add a border around a project without resizing

I'm relatively new to Photoshop CS2 and I'm trying to accomplish this: I want to put a thin line around the project, that basically looks like the border that Photoshop uses to demark the outer limits of the project. I've tried making a rectangle and then subtracting from it, but the sides are never exactly the same size. I'm sure I'm overlooking the obvious. I would like for this border/line to go on top of my existing work and not expand the canvas size. Can someone please help?

Thank you,
Paul

Select all with (black selection tool) on background, this should select the very edges of your document in one box; Edit, stroke, inside, 5 px or more depending on image res, hit okay, make sure it's stroking inside, not outside or it won't be there.