Hard Drive Preview / Disk Utility Used Space Discrepancy?
Do you have Time Machine enabled? This could be a result of the Time Machine snapshots...  

Have you looked at this thread here: How to fix Disk Utility, About This Mac, and Finder that disagree on hard drive space usage?

To disable Time Machine Snapshots : 

Open the Terminal from Applications/Utilities
Enter : sudo tmutil disablelocal
Enter your password at the prompt