Virus Scanning Program
The possible options for this field are :
Standalone scanner (clamscan)
This is typically the default, and will tell Virtualmin to use the ClamAV scanning program that does all of the work by itself. This is certain to work, but can be slower than the next option.
Server scanner (clamdscan)
This option can be selected if the clamd virus scanning server is running on your system. It is generally superior, as the ClamAV virus database is quite large, and the server only needs to load it into memory once.
Other program
This option lets you tell Virtualmin to use some completely different program, such as your own wrapper script for ClamAV.