Buy a new HDD, install the new drive in your PC, install your copy of win8, use the product key provided to re-register it, and then install all of your software. Sadly there is no 'easy' way to do this, you are essentially starting from scratch.
If you do not own a copy of win8 (ie, you purchased a Dell or some other pre-manufactured box) then you will need to contact your device manufacturer for a set of restore discs. If you bought restore discs when you originally purchased your computer then they typically cost $20, but ordering them after the fact will typically cost you somewhere from $40-100. Still cheaper than buying your own retail or OEM copy of Windows 8... but not exactly cheap either.