Wireless network affecting an AV sender

[#1]

You could go down the heath robinson approach and play around. Its hard to explain without picture but if we have
A              C

B              D
Each being a unit sender receiver or network then if the network is A + B its not so bad as you just need to stop signals from the right if the network is C + B its harder. Lets assume it A + B for wireless and C + D for video.
First step is get the units as far away from each other as possible both in distance and also on channels that is a + b as far away as you can get from c +d. Get the wireless and video units as close as you can to each other.
Turn the network router antena on its side  so its horizontal and the pc's the same this gives it a different polarity which will help like TV antena
From each antena the radio goes out in a circle basically so now you hae one going up and down and the other side ways.
Now you can have fun if you put kitchen foil or a biscuit tine one side of an antena it will tend to stop signal going in that direction and increase gain the other way  like the back bit on the TV antena. You could play about for ages and if you get it right it could all work.
You need the foil between A + C  B+ D so the signals dont go that way. It doesnt have to be foil but that would work best furniture walls and any other obstetrical will help. I have done it with the old CB and tv's and that is a lot more power so it is possible but its often a very slight movement that can change everything.
Have fun or get a direction antena for the network

[#2]

Thanks Chilly, it was the £80 price tag (or lots of wire) that I was trying to avoid, but it looks like the only real answer.
Samuria - I'd love to spend a day playing with this (if I thought it could work)  but the flaw is that in some way, both signals need to reach my son's bedroom. 
Running the co-ax is relatively easy.  I may even be able to use some ducting that already provides the route for the waste pipes from the bathroom, but it's time, (and messing around inside walls) 
John

[#3]

You could try for a parabolic reflector 
Instructions and video at http://binarywolf.com/249/diy-parabolic-reflector.htm

[#4]

What an amazing piece of DIY 
I almost feel like giving it a try just for fun!  I'd probably have to fit it to the AV sender rather than the router, because of the number of computers used around different parts of the house though.
Thanks for the link.
John