You haven't said which CD is asked for. In both the following cases, you can sometimes just click retry and it will be accepted. Here are two solutions:

1. If the message is to insert the Windows XP SP2 CD-ROM and retry doesn‘t work, read this http://support.microsoft.com/kb/900910

2. If the message is to insert the Windows XP Professional CD and retry doesn‘t work read this, it involves changing the word 'Professional' to 'Home' in two files; Fp40ext.inf and Ims.inf. Have a look here http://support.microsoft.com/?kbid=897128