Matt,
I first downloaded the driver from the dell website. Then extracted the driver to a folder on the desktop. I used WinRAR to extract the driver files. You will need to used the 32-bit Vista drivers, assuming your using PXE 2.0.
I think the next step is what made the difference. There were some files that were extracted to the new folder that, I assume, are not necessary and cause the PXE configuration to not load the driver. I have not, however, tested this. But get rid of the PRO Unstl.exe, Readme.txt, and Release.txt files from the VISTA-x86 folder.
Try to do the "Have disk" method again to add the drivers to PXE (pointing to the new Vista x86 driver folder that was created.
Like i mentioned previously, i haven't tested this theory as...
1) Our PXE is Working now with E6420s
and
2) I don't want to break PXE :)
Good luck.