You can fix this by pressing Ctrl-F9 until it's at an acceptable speed.

Also, if you feel a little adventerous, open up dosbox.conf (should be in the same folder as dosbox in your start menu), and search around for the string "cpu=normal". Change that to "cpu=dynamic". That way, DosBox will use a special method of emulating the x86 archetecture, making it easier to run games at acceptable speeds, if they work in dynamic mode. So far i've experienced no problems with dynamic mode, but if you do, call me.