I have a 100Mbps lan network for several computers and can't usually get more than 70-80Mbps when sending files over the network. Is this normal or am I doing something wrong?

for your adapter advanced setting change connection type from auto to 100tx this will help network performance.

70-80 is about right. Your hard drives are the limiting factor in file transfers. You also have network overhead that use a little bandwidth. You will never get 100Mbps unless your running a Gig network.

Hit the nail on the head.

Just FYI: When transferring files, you'll notice that you probably get the most bandwidth usage when you're transferring large files that don't constantly require the little Windows handshake crap between each file. Lots of small files require this handshake authentication between each file which is going to cause a pause and a drop in speed.