A system and method to securing a computer system from software viruses and other malicious code by intercepting attempts by the malicious code to write data to a storage medium. The invention intercepts the write access requests made by programs and verifies that the program is authorized to write before letting the write proceed. Authorization is determined by using the identity of the program as a query element into a database where permission values are stored. Depending on the presence or value of the permission value, write access is permitted or denied. Permission values can be set by the user, downloaded from a central server, or loaded into the central server by a group of users in order to collectively determine a permission value. The interception code can operate in kernel mode.