Can you turn you MTB into a good, road worthy machine? Short answer: Sure, why not?
If you intend to do any road racing,forget about it, HOWEVER, if having a quick and sturdy bike for commuting, weekend terrorizing, or even the occasional century is what your after, then definitely go for it! I did!
First and foremost, skinny tires are a must. There are lots to choose from, but I'd stick with 1.5 or even skinnier. Slicks are faster, but flat easier. Tires with kevlar belts are nice, but the belts will slow you down a little bit. The Conti Gatorskins get great reviews but are pretty expensive. In any case, a slick or semi-slick will get you scooting along quite nicely. Your choice.
Flat handlebars are nice for a short while, but get old fast. Bar ends are a nice addition and they make a bunch of different types, but I use an old Scott AT2 (I think, maybe an AT4) which is sort of a mountain bike aero kinda thingy. Looks a bit wierd, but works great! Scott doesn't make them anymore, but I see them on Ebay all the time. Any type that will allow you to change hand positions are good.
If you have suspension forks, either dump them or find a way to make them rigid (removing the spring on some forks and putting in a equal length of PVC pipe works). The up and down movement of the forks will rob you of pedal energy. For short hops is doesn't mean much, but on a century at around mile 80, you'll wish you had changed it out. Besides, it's kinda heavy.
If you want, change the freewheel/cassette to road bike gearing. Not very expensive. You won't get the same effect as a road bike, but close enough for not much money.
You can try to lighten the bike up by carbon seat post, stems and even a carbon mountain bike fork, but the costs can start to pile up, and when that happens, the fun starts to lessen. I use my budget MTB/Road UAB (Urban Assault Bike) for just about anything and everything. Finishing a century is an especially rewarding experience....seeing the faces of those on $2,500 road bikes when I ride up....priceless!