Converting bike from rim brakes to disc brakes
Your frame and fork already have disc tabs, which is the first step.  The second is a disc wheelset, which it does not appear that the stock wheels are.  Disc hubs come in two varieties: the common six-bolt ISO and the proprietary Shimano Centerlock, the former are identifiable by a large six-bolt-hole protrusion on the left (non-drive) side of the bike.  The latter have a rubber cover over a smaller splined mount.

Once a new wheelset is selected, you'll need a set of calipers and disc rotors.  The Avid BB7 is a popular mechanical disc caliper, and will work with your existing linear-pull brake levers.

Installation is fairly straightforward, but since it involves braking it's worth involving a professional if you're at all in doubt of your mechanical aptitude.  The disc rotors bolt to the wheel, the calipers to the frame.  The old v-brakes are removed.  New cables and housing are routed from the levers to the calipers and the calipers are adjusted.

Cost-wise, if you're happy with the bicycle otherwise it's not a poor investment - good wheels and brakes can be moved to future bikes if you later decide to upgrade the frame.  The ideal time would be to consider when the wheels already need to be replaced due to wear, since the new wheelset will represent the majority of the upgrade cost.