Provided are methods, apparatus and computer programs for identifying matching resources (data files and executable files) within a data processing network, by comparison of hash values computed for each of a set of resources. A match between a newly computed hash value and a previously computed hash value for a resource indicates that the resource has not changed since the previous computation. A match between hash values for different resources indicates that they are identical. The result of the comparison can be used to determine whether a virus scan is currently required for a resource, on the basis that a resource which is unchanged since it was classified virus-free remains virus-free and a resource which is identical to a virus-scanned resource does not require duplication of the virus scan. The methods, apparatus and computer programs enable more efficient use of antivirus scanning or management of a backup copy process.