In this case, it would be helpful if we knew a little more about your computer, such as CPU speed, amount of RAM, make and model of the soundcard (or chip).  Are you running Windows 7 on an older computer?  The first thing to do is to make sure your audio drivers (soundcard, if applicable) are the most recent.

It's also difficult because we don't know just how big your arrangement is.  Do you have a number of objects or tracks with time-stretching or pitch effects applied?  If so, you may be overtaxing your CPU..  If this is the case, you can export the effected objects and re-import them.  But then, of course, you're pretty much locked in.

With that said....

You can also increase the buffer size.  You may want to open the owner's manual and do a search for "buffer".  Read each instance where the word buffer appears.  That should give you a better idea of what's going on.

Go to the Program Settings ('Y' key), Audio/MIDI tab.  Look for "Audio Buffers".