Willow, is it a speedtouch modem? Basicly this is how it *should* go, as it went for me..
On the day of the activation BT disconnected my line and connected me to Bulldog. Up to then my current adsl worked fine, right up until the moment BT disconnected me. Before the activation date, make sure you print out a copy of the email from Bulldog which you have either got or will be getting, explaining settings like username and password...
When the day comes, all you need to do is put the cd you got with the modem into your pc, follow the installtion instructions onscreen and plug in your modem when promted. You'll then need to enter the username and password to connect. Hopefully it will all work fine tho my connection was a bit dodgy for the first few hours as Bulldog are still testing.
ps. My activation was done before 9am on the day of activation, but it might be longer, you just have to hope
Best way to check if it's been done is to see if you can still connect to AOL I guess...
I hope i've explained that well enough!