Looking around online, this seems to be an issue with the game's DRM (SecuROM, you monster).
Wikipedia Wrote:Known problems
- SecuROM may not detect that the original game disc is in the drive.
This can occur on virtually any configuration, and reinserting the disc
or rebooting the computer usually resolves the problem.
If that doesn't seem to work, disable any "questionable" software (CD/DVD burners, emulators) or antivirus/firewall (remember to turn them back on after trying this) you may be running and launch the game again.