Normally, when you hit F12 to get to the boot option, the system will wait for user intervention before going to the next step. i.e. F12, System will wait, Choose Network (PXE) option. For the Lenovo Workstations, the BIOS will boot into Windows if you wait too long in the Net Boot option. i.e. F12, System waits for 30 seconds and if there is no user intervention, it will boot straight into the local install of Windows.
Why would this be an issue? You can't batch turn on a bunch of computers to quickly image them.
Scenario 1 with HP & Dell Workstations
Turn on Computer 1, F12
Turn on Computer 2, F12
Turn on Computer 3, F12
Turn on Computer 4, F12
Turn on Computer 5, F12
This can take a couple of minutes but you then loop back to Computer 1, you can choose the Network option and the Computer will connect to the imaging server to get a default Windows 7 image loaded.
Scenario 2 with Lenovo Workstations
Turn on Computer 1, F12, the system boot into default Windows if you don't choose Network within 30 seconds.
This workflow requires you to be at the workstation during the entire boot process.
For the Lenovo Workstations, just unplug the keyboard before turning on the computer and it will automatically boot into Network mode, then plug-in the keyboard afterwards.
Below is another post about various computer configuration issues dealing with keyboards, mice and monitors: