You are not being very helpful to help yourself
How do you access BT Yahoo, which is an email server, from your PC?
If it is using your PC's browser, then you are accessing it via webmail and it has nowt to do with your PC or Windows, and the address book will be on BT's server not your PC.
Otherwise you are using a program, an email client like Outlook, on your PC. If so, please tell us which one?
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 20 Meg WBC