When I insert certain CDs into the computer, Windows Media Player does not recognise the CD so I cannot rip tracks off the CD. Instead, a small window appears like the CDs own in-built player. How can I rip tracks off the CD? The computer is running Windows XP with Windows Media Player 10.

What you have encountered here is a form of copy protection on music CDs. Some labels have protection on the CD so they can only be played through an embedded player which is on the CD itself. This would explain why a small window opens, which you said appeared to be like the CDs own player. It would also offer an explanation as to why Windows Media Player will not recognise the CD. There is a lot of information on the internet about these copy protection schemes, and their apparent incompatibilities with some devices (on the cover of your CDs you may notice there is a list of systems with which the CD is compatible). Since the music label has gone to this length to institute copy protection on the CD, I would advise against attempting to crack or bypass the protection since that would likely be in contravention of relevant laws and licensing agreements.