I am using my computer about 40 metres away from the Thomson5BF744 modem supplied by Telecom, at this distance I cannot connect to Internet, I can connect to Internet within 15 metres just with low signal strength, is there a way to increase the range of the modem ? I was previously using a Dlink modem with a 5 db gain aerial fitted and getting good reception at 40 metres. It doesn't look like the aerial will fit the Thomson modem.

You could do the job properly with a pair of these as a bridge between the two buildings;
http://www.tp-link.com/lk/products/details/?categoryid=4040&model=TL-WA7510N
Or you could buy a pair of these and keep it all inside the buildings;
http://www.tp-link.com/lk/products/details/?categoryid=1247&model=TL-WA830RE
You should always wire the Access Points into the router when possible. If you place the devices in the window of each building where they can see each other then they will work. The trick with WiFi is if you can see the other end then it *will* work. If you can't see it then it will work a bit or not at all.

Why don't you buy a box of CAT5 cable and just run a really long wire? It will work a lot better and cheaper. You could have one AP at the far end so you have WiFi there.

You can use repeaters which you can place midway between you and the WiFi access point.

Take a look at http://reviews.cnet.com/4520-11334_7-6271405-5.html to have an idea of how it works.

With strategically placed repeaters, you can cover very large areas efficiently.