If you're just doing it for fun, your road bike should work adequately. What you need to do is change out the wheelset for something a little hardier, and put some cyclocross knobbies on your wheels (remember that they still have to fit in your frame, with a bit of clearance). Also a good idea is to exchange your dual- or single-pivot road brakes for some good, old-fashioned cantilever brakes; the cantis have a lot more clearance for mud and such. You may want to throw on a smaller crankset too. And finally, keep that bike clean; thoroughly wash it after every ride. Nothing breaks a bike faster than mud and grit riding around its frame and components. Pretty much the only difference between converted road bikes and dedicated CX bikes is the geometry; CX bikes have a bit more relaxed geometry, and a bit more clearance between the two sides of the fork and the two sides of the chain and seat stays.