Need MDT task customization help plz

Okay so my situation is this:

Trying to use MDT 2012 Update 1 to create automated installation source to run from a USB stick.

Installing Windows 7 Enterprise VL, along with Off 2010 sp1

Installing to Dell Optiplex and latitude systems.

Fell free to ask if you need more info.

Problem is this.

I want to do a clean install on the systems, but I want to leave the Dell OEM partitions in place.

So I need to have the Lite touch or eventually no touch install, format and install on Disk 0 Part 3 and leave the other 2 partitions alone and not assign drive letters to them.

So in the MDT task sequence, I don't understand how I am supposed to coordinate the settings under "Preinstall - New Computer only - Format and Partition Disk" and "Install - Install Operating System."

Can I just disable the "Preinstall - New Computer Only" section and let the install section do what I need?

I've spent a few hours searching through other posts and other sites trying to find a clear answer and I can't seem to find something that answers my specific situation, so maybe I am asking the wrong questions.

I'm new to these tools and trying to learn as I go, so please don't assume I know anything.

Thanks!