A security module for a pay-per-use computer supplies an appropriate BIOS for a given mode of operation. A power manager in the security module powers only essential circuits until the BIOS is operational to help prevent substitution of a non-authorized BIOS. The security module also includes a capability to monitor and restrict data lines on a bus between a main computer processor and computer system memory. When the computer is operating in a restricted use mode, data lines may be restricted to allow only minimal access to the computer system memory. Bus transactions may be monitored to ensure that only valid transactions are occurring and are within the designated memory space.