I would imagine that it's in the Registry - don't know exactly as I don't use BT.
If so,
Start
Run
REGEDIT
Once loaded do CTRL-F (find)
Type in the URL of the page the dialler takes you to. Press ENTER
When it finds it (if it finds it) replace with the URL you want. Press ENTER
Leave REGEDIT
- done
Warning: Don't start mucking around with the Registry if you don't know what you're doing - you could wreck your machine!