Aruffell, not meant in an offensive way at all, but go have a quick read of what ADSL is and how it works, will make a lot more sense to you that way.
Fornow think of it like this:
It used to go:
1)Virgin line in
2)Splitter (on the wall) for TV/Internet
3)Modem connected to Internet side of splitter
4)Computer connected to modem.
It now works as:
1)BT line in
2)Microfilter (splitter) in the phone socket or a filtered master socket (these are usually kinda crappy by the way).
3)Modem connected to the Internet side of the splitter
4)Computer connected to modem.
If you want to make more than one computer have access you have to go AFTER the modem. Easiest way is to replace the modem with a DSL router (make sure it mentions dsl as there is a different type for cable/virgin) then CAT5 from computers to the router. Stick in the username/password into the router and it will (usually) work out the rest. CAT5 can be replaced by wireless to the router or partly replaced by powerline/homeplug etc (powerline/homeplug is networking over the mains cables and can be googled i'd recommend if you don't want to run cables around the house, buy a set/pair and they will work out of the box).