Cannot install Network Controller/Ethernet Controller for Optiplex 980

I had to format and reinstall Windows 7 Ultimate 64 bit on an Optiplex 980 desktop PC. After the installation, all drivers were installed according to Device Manager except the Network Controller and Ethernet Controller. I downloaded the appropriate files from the Dell web site, but when I try to run them I get an error that says "Invalid self-extractor configuration. The file may be corrupted."

Then I downloaded the 64 bit Network Adapter Driver for Windows 7 (PROWinx64.exe for Intel 82578DM Gigabit Network Connection) from the Intel web site, and when I try to run that it says "This version of this file is not compatible wit the version of Windows you're running..." but I am running 64 bit Windows, and the 32 bit version doesn't work either.

I've had this same problem with reinstalling drivers on Dell laptops but have always been able to remedy it by using the original driver CD, and usually installing first the Network Diagnostic driver. Unfortunately I don't have the driver CD for this machine.

I don't want to have to trash this machine, but it's obviously basically useless without networking capability. Any help would be greatly appreciated.