[CamB]

You need disc mounting tabs on the fork and on the frame.

If they're there, it is a piece of cake. If they aren't there, a new fork (with the tabs) solves the front (although if your bike has a 1" head tube you are out of luck), but nothing short of a trip to the local welder (which is not advised if the frame is alloy and too expensive to be worth it if steel) will work it out. On top of that, you need hubs which accept discs...

So in your case, the brakes are just the tip of the iceberg. On my wife's bike, they'd just bolt on (it is designed that way - it even has disc hubs) - but its a problem for you.

However, the good news is that a whole new Hard Rock with discs is only $440. Mind you, the spec is pretty low on that (wouldn't stand up to extended rough usage). The Rockhopper is much better (at twice the price of course).

(edit) Note there is a difference between mechanical and hydraulic disc brakes.