Difference b/w mountain bike and road bike

BobJ has covered it mostly already.

If nothing else, put some road tires on the MTB and see what happens. The tires will make a big enough difference for you to notice.

After that, the fit, geometry, aerodynamics, and weight savings will help add the speed to your rides.

Also, do you use pedals, cages, or clipless pedals w/ the MTB? That will also help with the biking effeciency too...

If you are planning on getting a road bike after your first tri, don't invest money in your mtb to be a little faster.

Sure putting slick tires on your current bike will make a difference maybe 1 mph but is spending $50 worth it. Keep the $50 and have that go towards your first road bike.

Getting a well fit road bike will make a huge difference and  you could easily gain 2-3.5 mph or maybe more.

Go into your first tri to learn, have fun, become a triathlete and enjoy yourself.

Don't get caught up in paces, placement and times...you have the future to worry about those type things.

I missed the fact that the OP's race is in just two weeks.  But still, I'd invest $25 and get a set of these: http://www.performancebike.com/shop/profile.cfm?SKU=15304&subcategory_ID=5425

The road bike is WAYYYY faster, unless you have a lightweight racing mtn bike (<25 lbs), and even then, the road bike is still faster (but not by as much.) Most mtn bikes weigh 28-32 lbs vs road bikes at 17-20lbs, and then you throw in bigger gears, thinner slick tires, and aerodynamics, and there's really no comparison.

Going from mtn bike to road bike is one of the few truly game-changing equipment upgrades that you can do in triathlon. You can literally jump from MOP to FOP or BOP to MOP with no added training just by using the road bike. For me, riding the same roads, I went from 15-18mph to 22-25 mph instantly. As you might imagine, I went from barely being able to hang on the back of the pack even with drafting, to going fast enough to drop the entire group I had been riding with, and pretty badly at that. My steel mtn bike weighs 36-37 lbs though, so you can imagine that would happen going from that to a Cervelo P2C with aerobars!

Unfortunately, after that, gains are small. Even those oh-so-sexy aerobikes with fancy aerobars and special forward-seat positions that you see the pros using will give only a few percentage points advantage vs a standard road bike of similar caliber.

I'd recommend just going out to your tri and having fun on your mtn bike - you'll still pass a bunch of folks on it. However, if you suddenly get the urge to really see how well you can do in the next 2 weeks (not necessary, in my opinion), rent/borrow a road bike, and that will have the most dramatic improvement on your race performance.

Me too! I got a second hand B2 felt, it's like a totaly different bit of gear compared to the usual mountain bike, having never have ridden a road bike I had no idea what I was in for, pretty twitchy though hey

good luck with your races and have fun

Depending on your situation, I'd say save your money on slicks and invest in a road bike.  I'm new to the sport as well and don't have unlimited resources so I'm definitely constantly on the lookout for deals.  I found my bike for 450 on craigslist.  $1000 bike with around 100 miles on it.  It looked almost new (had some dirt on the tires and a small tear in the bar tape...thats about it).  You can really find some great deals if you look hard enough.  That 50 for slicks goes a long way towards $450.  Just my .02.

Good luck at your race!