Hi stephjh. This kind or problem can arise if you have a motherboard with onboard LAN. If this isn't disabled in BIOS, Windows installation can try to install Microsoft Network, but doesn't always load all the files.
You can extract the required file from your Windows CD using Start, Run, and type in the box "sfc" (without the inverted commas). Then select "Extract one file from installation disk". The file should end up in the Windows\System folder.