fta123: Welcome to the forum.
Is there a way to control these devices via computer?
Yes. I suggest you look at X10's ActiveHomePro. They regularly have it on their website's deals page for $50 and it includes all 4 software plug-ins.
...the shed is about 40 feet from the back door of the house so distance might be of some concern.
If this line is on a separate breaker and is part of the main house electrical service, then it should not be a problem.
I have just bought some X10 wall switch modules...
What are you planning to use for controlling the wall switches? Did you also purchase a plug-in controller or transceiver / remote?
What about having the front lights turn by using another X10 motion sensor?
To do this, you need a Transceiver that receives the X10 RF signal and turns it into an X10 Power Line Control (PLC) signal.