I'm wondering if, first, if the Windows Server 2003 GUI looks like Windows XP? Second, do regular programs that run on XP also run on Server 2003? Or are there special programs written for them? How does all that work? Thanks!
2003's GUI is the same as XP's 'Classic' GUI. It's possible to hack on the uitheme.dll 'shiny stuff' to 2003, but not often done. This gets better in Windows Server 2008 as you can install the 'Desktop Experience' option and get a desktop similar to Vista.
2003's program and driver support is pretty much in-line with XP. Most printer and device drivers that work on XP will also work on 2003, with a few exceptions. Programs work in almost all cases.