For protecting a computer from manipulation of register contents, copies of registers to be protected (6; PC) are created in separate redundancy registers (10a-10c). At each instruction execution, the content of the register to be protected is compared with the copy thereof. The instruction is only executed if there is a match of register contents. If there is a mismatch of register contents, this is interpreted as an indication that the content of the register to be protected has been manipulated, and error handling is performed.