Windows XP keeps a folder of files that are used when the computer starts and when applications are started.  This speeds up the starting of applications, but it also takes up drive space and system resources.  This folder can become bloated with files that were used in the past but are no longer used.  This is why it is recommended to empty this folder at least once a month.  Files that are still in use will be replaced.

If your version of Windows does not support the PreFetch list, this list will be empty and the dropdown box will be disabled.

PreFetch constantly monitors what is running on your computer.  This monitoring can slow down your computer.  If you do not want PreFetch to monitor your computer, you can either select Disable or Boot Only.  Application Only or Both Application and Boot will cause PreFetch to constantly monitor what is going on and will make your applications to start faster.