Wow, I just downloaded x-com again, but couldn't get a grip on dosbox. Try Jagged Alliance 2 if you haven't.
I downloaded DOSbox a few days back too. If you open it and type "intro" you will get a lot of help texts and see some useful commands that can be run manually. But you can also edit the 'DOSbox.conf' file (in notepad or similar) and at the very bottom under [autoexec] you can type in what commands it should run on startup. I only use DOSbox for MoM at the moment so these are the commands I added:
mount m c:\games\master~1
@echo Run 'magic' for Master of Magic
(keyb sv - gives me swedish keyboard layout instead of the default US one, because our fingers up here are all bent and knobbly from fighting with the polarbears, so we need this...)
(mount m c:\games\master~1 - makes it possible for DOSbox to find the folder where the game is at, without this it just don't have access to anything on your computer at all. I picked m only because MoM starts with an m, z is already used by DOSbox but all other letters are available (it is an emulated environment, no conflicts will occur from using driver letters like c or d if you choose to), c:\games\master~1 is the name of the folder, in XP it looks like 'c:\games\master of magic' but DOS only handles 8 characters so the fancy long folder names can't be used)
(m: - the game folder is now accessible, this command moves me there)
(@echo is just to show a message on screen, in case I feel lost or drunk. I could have written 'magic' instead and have DOSbox start the game automatically, but prefer this way)
Also, in order to get MoM to run a bit smoother on my machine I changed the emulated cpu speed under [cpu] by changing:
I should really increase this number though, it makes the game a bit slow. The cycle speed can be increased inside the game by pressing 'ctrl-F12'. each time will add another 500 cycles. Different games might prefer different cycle values for best performance.
Hope this helps, good luck
(edit: small cleanup)