Another completely green dude who completely skips the readme... Read it, and better do it now.
The official DosBox FAQ, and particularly
this part of it are next in order.
And FYI, DosBox doesn't have a fixed speed. Try the following:
frameskip=1
core=dynamic
cycles=10000
cycleup=1000
cycledown =1000
If you still don't know where these go,
read up on the config file.
Then tune the DosBox cycle value using CTRL+F11 and CTRL+F12 to the maximum your machine can handle on that game. To check that, enable XP's resource monitor thingy and minimize it. Go into DosBox (fullscreen mode! that's important!) and after a minute or so back to the resource monitor. Did the CPU usage reach 100%? It should be as close to the top as possible without actually reaching it.