Funny problem: Set the fixed IP of a network interface - once you have clicked OK back out of the network interface properties dialog box, it reverts to an automatically assigned IP of 169.254.x.x. Go back into properties for the network interface, and IPv4 settings still holds the static IP you set. It is not possible to ping the interface on that address, and ipconfig shows only the automatically assigned address.
I had this problem on a Windows Server 2008 R2 today, and spent a fair amount of time troubleshooting it, including running Windows Update, several reboots etc..
Solution: In the end, I uninstalled all network interfaces found under Network Interfaces in Device Manager (right-click Computer > Manage > Devices) and then right-clicked the device root and selected "scan for hardware changes" to automatically reinstall the network interfaces. Voilla, the IP settings now sticks and actually works!
Possible reason: I am not sure what messed up the system, but my main suspect would be a special high performance frame grabber driver from Pleora that was installed on this server for two other network interfaces (not the one I was trying to set the IP on). This driver obviously digs deep into the device hierarchy, because it removes the network interfaces it controls out of the Network Interfaces tree entry in Device manager over to a new tree entry called "Pro/1000 grabber devices".
Anyways, I got my solution - now it works!