What error are you getting?
Do you have a different optical drive you could try the CD in, or even a different computer?
Are you shutting down all non-essential programs (especially anti-virus) before trying to start the game?
If all else fails, in the error message (either 'disk detection' or 'no disk found'), click the 'Information' button, then when that finishes, click the 'View Report' button. Save the resulting text file somewhere, optionally zip it, and email it to Larian (support@larian.com) with a description of the problem. They can create a file the copy protection can use to check your system on startup, rather then the play CD.
