A computer system contains selectively available boot block codes. A first boot block is of the conventional type and is stored in storage media such as flash ROM on a system planar with the processor of the computer system. A second boot block is located on a feature card and contains an immutable security code in compliance with the Trusted Computing Platform Alliance (TCPA) specification. The boot block on the feature card is enabled if the first boot block detects the presence of the feature card. The computer system can be readily modified as the computer system is reconfigured, while maintaining compliance with the TCPA specification. A switching mechanism controls which of the boot blocks is to be activated. The feature card is disabled in the event of a computer system reset to prevent access to the TCPA compliant code and function.