1. A computer program product embedded in a non-transitory computer readable medium, the medium storing a Digital Right Management client system including a plurality of Digital Right Management clients, the Digital Right Management client system comprising: a virtual Operating System managing unit stored in a memory that separates a kernel of an actual operating system installed in the Digital Right Management client system to generate and manage a virtual operating system; a branch process information managing unit stored in the memory that manages branch process information according to a type of a document that a user attempts to read; an application program branching unit stored in the memory that analyzes the branch process information and executes a Digital Right Management client agent for managing a Digital Right Management client in an actual operating system region or a virtual operating system region according to the type of a document that the user attempts to read to allow the user to read the document; wherein a number of virtual operating systems to be generated corresponds to a number of Digital Right Management clients installed in the Digital Right Management client system; wherein the branch process information includes first information for determining a document type according to a document encryption method and second information for determining an execution operating system region in which the Digital Right Management client agent is executed according to the document type; an inter-process communication interface that performs communication between Digital Right Management client agent processes that are executed in the actual operating system region and the virtual operating system region, respectively, according to the type of the document; wherein the plurality of Digital Right Management clients include a first through an nth Digital Right Management clients for reading a first through an nth encrypted documents, and wherein the virtual operating system managing unit generates (n−1) virtual operating systems according to the n Digital Right Management clients (n is an integer same with or greater than 1); and wherein the application program branching unit executes the Digital Right Management client agent in the actual operating system region when the document that the user attempts to read is a first encrypted document, and the application program branching unit executes the Digital Right Management client agent in the (k−1) virtual operating system region when the document that the user attempts to read is a kth encrypted document (k is an integer same with or greater than 2 and same with or smaller than n).