So much misinformation.
Depends on your frame. You can buy slicks or semi-slicks for your 26 inch wheels, put on a road cassette with a couple spacers, put on some drop bars (or keep your MTB bars).
*some* mountain bike frames will accept a 650b, but rim brakes won't work because of the spacing...so you would be building a custom wheelset. The note about dropout spacing being different is correct. Mountain bikes use 135mm rears and roadies use 120. Some old school MTB frames will take a 120 rear hub.
The best way to turn your Alum MTB for road use is drop as much weight as possible from it and put on slicks and some smaller gears, and larger chainrings. One other option for streetworthy bike on a budget....singlespeed or fixed gear.
It would be a bit of a frankenbike, but what the heck? I ride an old MTB converted to street use because our bike path system doesn't work well for conventional high pressure/low volume roadie wheels. Too many driveway/curbs/bumps that would ruin a road wheel.
You want to see how an OEM does exactly what you are looking at? go to cannondale.com and look up their badboy lineup. More of an urban assault/commuter bike.
The fact is that with some jury rigging, you can make whatever you want to with your old frame....but the chances are that it won't accept the diameter of a 700c wheel. You can make up for it with 26", 650's and the right gearing and tires.
Check out the source section. Most of these bikes are either mountain bikes turned into cyclocross or road bikes turned cyclocross. It takes some ingenuity, but with time and money...anything is possible.