It took me a few days to figure out a solution, so I thought it's a good idea to publish it.
We have several Dell Optiplex 7010 machines in use. They are running Windows 7, and have an onbard Intel 82567LM Card, attached to a Dell PowerConnect 5548 Network Stack. Most – not all – of the machines appeared to have problems with their network link while booting up. As a result, they were not able to fetch Group Policies, Scripts, and other Settings.
Several Entries could be found in the System Event Log…
*Source e1cexpress, ID 27: Network link is disconnected
*Source NETLOGON, ID 5719: computer was not able to set up a secure session
*Source GroupPolicy, ID 1129: processing of Group Policy failed because of lack of network connectivity
After installing a lot of Network Card drivers, Bios/Firmware Updates, tweaking Switch Settings, working through Microsoft KBs (KB202840, KB2459530, KB938449) and much more, I finally found an workarround.
1. If your switch supports SpanningTree Protocol, make sure you enable the portfast option for the corresponding Switch Ports.
2. Install the latest Intel ProSet Software (includes drivers): http://www.intel.com/support/wireless/wtech/proset-ws/sb/CS-034041.htm
Do NOT use Dell Drivers
3. After installing ProSet, switch do Device Manager, open your Network Card Properties and set the Option "Wait for Link" to "ON"