1. They might have access to leaked phone number lists, or they may have a copy of a Directory Enquiries CD set from BT, or they might just make them up!
2. They probably don't. It's just a line dangled to make them appear more plausible. Alternatively, they may have some leaked information from BT or your ISP, because it is certain that at a known time, those organisations know which IP address is allocated equipment on which phone line.
3. Windows is ubiquitous. For home systems, chances are that at least 90% of homes with a computer have a Windows variant rather than a Mac, Linux or other system. And even those with Linux probably have Windows installed somewhere as a dual boot.. The Reg. readership are not typical. My house as all three (Win2000, WinXP, and Win7, OSX, and Linux), as well as an AIX box.
I suppose that there will be an increasing number of houses that have broadband for just their TV, gaming console, iPad or Android Pad. I wonder how the ISP's will cope with supporting such customers? At the moment they all appear to be geared around having a Windows box around.