Open the box, remove the hard drive & store securely as a back-up of your data at time of transfer to the new system.
Once you're happy it's no longer needed
then think about disposal. Darik's Boot and Nuke is good for overwriting an entire disc:
Darik's Boot and Nuke - Wikipedia, the free encyclopedia
Use a gas axe or drill press to put a hole right trough the drive if you really must physically destroy it.