Power point, excel, all MS Word programs have "shapes", which are easy to use and can be copied / pasted into other Office Programs. If' you're going MS Office, pick Visio - it is designed for visual representation (also I'm going to marry it). Visio has a tool that lines up shapes and lets you edit arrow types etc, while on other Office programs you have to get "creative" to get there.
I like Open Office Draw, it is an open source alternative to Visio (free). That also means it has weird bugs sometimes and doesn't have all of the functionality of Visio. But the community is great and chances are it'll meet your needs if you already know Visio. Free. Best for the adventurous.
Excel can do many things. But that doesn't always mean it should do those things. For example - I have a warehouse manager that built an entire factory map, in Excel, by making each cell represent one 1/2in of the factory. It was a remarkable triumph of using the wrong program. You could tell hours of work and manipulation went into it. It was 26 by 50-some pages printed. But with Visio, you just type in your scale, and enter h &w, and an object the right size pops up.