Assuming you mean during playback, you may need to increase your buffers.  They can be found in the Program Settings on the Audio/MIDI tab just under the drivers section.  Try increasing the Multitrack buffer a notch.  Don't over-do it. You can cause excessive time delays.

"No other programs are open."

Well, that may be partly true.  There's always something running.  It might also be a good idea to set MMM as a priority.  In the Task Manager, in the Processes, find the .exe file for MMM, right-click on it and increase the priority.

Also, anti-virus programs can sometimes be troublesome, especially when they tend to download updates at the most inopertune times.  If you can completely sever your internet connection (unplugging the ethernet cable is a surefire way), turning the antivirus off can help lessen problems. (Just remember to turn it back on and plug back in before you reconnect to the internet.)

If you have at least two hard drives, it's probably best if you leave the program on your boot drive (usually the C: drive) and store all of the soundpools and other media on a second hard drive.  That way you don't have only one hard drive struggling to run the program and trying to play various media files simultaneously.  (Yes, I know it's actually run from the RAM, but this will reduce access times.)