A computer system that contains a boot signature checker is disclosed. In one example, the boot signature checker monitors the bus connecting the processor with the memory and provides a boot signature that is a function of the signal sequence detected on the bus. If an unauthorized user tampers with the operating system within the memory as when attempting to gain unauthorized access to presentable content such as television programming, the boot signature checker will in all probability produce a different boot signature than expected. The decrypter will respond to this different boot signature by, for example, deactivating thereby potentially preventing an unauthorized user from gaining access to presentable content.