The arduino playground Arduino Playground - InterfacingWithHardware has a link to a diagram that shows you all you need to know about controlling a DC motor using a L293 IC. You sound like you know a bit about PWM - shouldn't be hard to get it working using the example (OK it's in German - but you'll get it)
http://gestaltung.fh-wuerzburg.de/blogs/exint_ws06/?p=505
The datasheet for the L293 is useful too