If you use Hyper-V and still need to install a Windows 7 or Windows Server 2008 R2 OS then you need a Generation 1 virtual machine. If you do the installation via network boot you might find that, compared with Gen 2 VMs, there is a catch to make this procedure work.
PXE on Gen 1 VMs is not possible with the default network adapter used when creating this type of machine. The reason is that this is a synthetic network adapter that does not have a boot ROM capable of taking an IP address from DHCP and downloading a boot file. PXE can be used by a Generation 1 virtual machine by means of the Legacy Network Adapter which is an emulated hardware device.
So the solution to this problem is to just remove the default Network Adapter and add the legacy one. This can be done from the Add hardware menu option.
This is how such a network adapter look when configured:
One thing to not forget is the boot order for the virtual machine. Set the Legacy Network Adapter first in case you need to (for example if you already have an OS installed and the HDD is first).
With this setting made now the virtual machine can receive boot images by PXE: