1. A method of controlling write access to a mass data storage device by: running a first process that operates in conjunction with an operating system that manages access to a data storage device, said process operating in kernel mode monitoring data storage device accesses; detecting by use of the process an attempt by an application distinct from the first process to write data to said data storage device; in response to such detection, interrogating a rules database wherein said rules database is comprised of a plurality of references to a corresponding plurality of applications, each reference further comprised of at least one access level associated with the corresponding application; and controlling write access to the data storage device by the application attempting the write in dependence on a result of said interrogation; wherein said interrogation result is comprised of the value of the access level associated with said application.