I started to get this message this week when booting, so I'm assuming the NIC is toast. I'm planning to add a new card to replace it, but there is no option in the A05 bios to disable it. Is there a jumper or how can the onboard NIC be disabled?
According to the on-line version of your model's Service Manual there is an Network Interface Controller option under the Integrated Devices section of the BIOS Setup. That option should allow you to turn it Off. Click here for the on-line manual I am referring to.
It wouldn't be the first time I've seen the documentation in error. The only other thing I can suggest is try posting your problem in the Workstation / Optiplex forum. That forum may provide a better answer from other owners of your model. Click here for a direct link to that forum.