A system for scanning computer files for unwanted properties, such as containing computer viruses or being spam e-mail, allocates a priority to pending scan requests based upon the identity of a computer user associated with the scan request. In the case of a normal file access request, the computer user associated with the scan request may be the file access request or in the case of an on-demand scan, then the computer user associated with a particular scan request for a computer file may be the owner or creator of that computer file. In the case of scan requests associated with e-mails, the sender or recipient computer user may be used in the allocation of a priority level for the scan request.