That looks like a threaded stem, but here's an article about threaded versus threadless systems:
http://sheldonbrown.com/headsets.html
As mentioned, most forks these days are threadless. If you have a threaded fork and want to put on a threadless fork you'll need a new headset and stem. Of course, make sure the fork you're looking at has studs for your V-brakes.
Rigid forks meant to replace suspension forks are said to be "suspension corrected" and have a larger axle-to-crown measurement. That's the number you want to look at. The distance from the axle to the fork crown - the point where the fork disappears into the headset. Contrary to what many believe, I don't think this number is all that critical. Your bike doesn't have much travel, even if you put on a non-corrected fork I doubt it would change the handling much. I've done these sorts of swaps before with no issues.
Finally, cheap suspension forks suck. A rigid fork is much better than a cheap suspension fork especially for commuting. A suspension fork is really not necessary for commuting, and I think you'll appreciate how much lighter and more efficient your bike will be with a rigid fork.