Originally Posted by
DiegoFrogs
I think a high-end, rigid, probably late 1990's MTB with slick tires and road gearing, possibly with a different handlebar setup, could be a good place to start.
I also like old sport-touring bikes for this purpose. But I rarely ever ride fast.
Mostly I would just avoid suspension, use a reasonable tire of whatever size you like, work out the appropriate gearing and an aero hand position, however you choose to do it.
+2.
Put slick tires on your MTB and you'll notice a world of difference. Suspension is anther thing that can suck up energy (if you have front suspension, replacing it with a solid fork is a very cheap and undoable upgrade.
Putting on bar-ends so you can move your position to be a bit more forward and aero can help as well (and clip-on aerobars although somewhat silly looking on a MTB can work here as well).
To the OP, almost all of the fastest riders ride road bikes, but just because you ride a road bike doesn't make you one of the fastest riders. There are cheap upgrades that you can make to your existing bike that will make it a lot faster - riding it much more and getting in better shape is pretty much at the top of the list.