Companies like Symantec, McAffee, Trend Micro, Kaspersky, etc. provide enterprise-level malware protection. Choose a major anti-virus company and familiarize yourself with their product line.
Note: It is not necessary to diagram your complete network, just a high level representation of it.
For example, you could represent the 2000 Windows 7 computers with one Icon labeled Windows 7 Workstations (2000). However, if you include a security appliance that provides malware protection, it should be included as a separate icon. Also, indicate location of software components (clients, servers, databases, management tools, etc) on your diagram, as well.