Firstly, try removing the temp folder (back it up/copy it first if you're worried about data loss!) from your assets folder and re-starting Unity -- I've heard a number of times that similar problems have been solved that way. If it's still not working try the following:

1. Save any other work you're doing, and close all un-needed programs. You can keep a web-browser open with these instructions, as long as you don't have any Unity-related web-players or the like open. For best results, start from a fresh re-start of your machine.
   
   
2. Open task manager (press Ctrl + Shift + Escape, -or- press Ctrl + Alt + Del and choose "start task manager" from the menu).
   
   
3. If you have Unity currently running, close it. After waiting a moment, ensure there aren't any copies still running in task manager.
   
   
4. On the "processes" tab of task manager, look for and close any processes that look like the following: "x:\Windows\Explorer.exe /factory,{guid} -Embedding"
   
   
   
   • Note: to do the above you need to enable the "command line" column in View > Select Columns.
     

• Ensure you don't have any files from the _Data folder open in any program.
  
• Re-start Unity, and perform a build in an empty directory.
  Let us know if that helps at all! I had these instructions saved in a document after experiencing a similar problem a while back and piecing it together from a couple of different sources online -- hopefully it works for you as well!