Hi erasing the contents of 'free space on your hard drive' as well as giving you extra GB, security of erased documents etc, will it actually improve the performance of a PC? Also how often is it safe to do this?
Erasing the free space on your hard drive won't make any more available to you - it's already marked as free by the OS. It will improve security slightly, should someone else have physical access to your computer. It won't improve the performance. For most people it's only worth doing when you decomission a drive or a computer.
Of some importance of deleting the slack space of the hard drive is to make sure what you previously deleted is destroyed, not just overwritten. Privacy is important, likewise so is keeping private information private.