There are many things you can and indeed should do to protect your computer from malware and viruses. A chunk of crystal, however, is never going to cut it.
This one has been doing the rounds online for a while now, but it's probably worth repeating... if you're not giggling at the concept already.
So the theory goes, the "protective" nature of malachite crystals is perfect for keeping your computer virus free, because it will "soak up some of the electromagnetic pollution emitted by your computer".
Who needs expensive AV products, when a simple crystal can stop the malware?
Everyone, that's who, because it's absolute and total junk science, and even a single second's thought should reveal that unless you're entirely gullible.
Any crystal that could change patterns on your system storage isn't going to simply change the patterns that coincide with viruses and malware. It would have to emit a strong enough field to affect everything, from applications to operating systems, right down to the BIOS level. Frankly, anything radiating that much power is probably nothing you want to stand too close to.
How to: Protecting your PC from viruses with magic crystals [Graham Cluley]