Robert,
On our Website, I was able to locate an instrument driver listed for:
E3640A,E3641A,E3642A,E3644A
It is written for LabVIEW,LabWindows/CVI
You can find this at the following path:
ni.com>>Downloads>>Instrument Drivers>>Downloads>>Select HP and Power Supplies..>>Seek
It should be the last result. I believe this is what you are looking for.
Good Luck,
Chris Drymalla
Applications Engineer
National Instruments