If you are running VirusBarrier X6 and trying to install Mac OS X Mavericks but receive an error from the Mac App Store, it may be caused by the VirusBarrier Cookie Filter.
You can turn off the Cookie Filter in VirusBarrier X6. Launch VirusBarrier X6, click Surf at the top of the window, and click the Cookie Filter tab below. Slide the Cookie Filter to OFF.
Once you have disabled the Cookie Filter you should be able to download and install Mac OS X Mavericks. You can re-enable this feature once installation is complete.