http://xbcd360guide.50webs.com/guide.html
Follow the guide. There's a profile on the downloads page (the last one, I think) that has the triggers set up as digital buttons.
Basically, MS's drivers for 360 controllers "suck" because they want all the controllers to be mapped the same so they're compatible with Games for Windows titles. XBCD360 is a set of third-party drivers with a config panel that lets you remap the buttons and axis however you'd like. It works great, I use my 360 controller for emulators (it's awesome for SNES, PSX and MAME in particular) and used to use it occasionally for FFXI. You can also set up multiple profiles (for example, if some of your games need analog triggers) and switch between them using the guide button.