Systems and methods are described for allowing the execution of authorized computer program code and for protecting computer systems and networks from unauthorized code execution. In one embodiment, a multi-level proactive whitelist approach is employed to secure a computer system by allowing only the execution of authorized computer program code thereby protecting the computer system against the execution of malicious code such as viruses, Trojan horses, spy-ware, and/or the like. Various embodiments use a kernel-level driver, which intercepts or "hooks" certain system Application Programming Interface (API) calls in order to monitor the creation of processes prior to code execution. The kernel-level driver may also intercept and monitor the loading of code modules by running processes, and the passing of non-executable code modules, such as script files, to approved or running code modules via command line options, for example. Once intercepted, a multi-level whitelist approach may be used to authorize the code execution.