Hi Otte
In response to your question. You should have avast antivirus server edition installed on all your servers! simply scanning on-demand is not advisable.
If your servers where attacked by a network worm they would not be protected! your data could be corrupt, compromised and worse still nonrecoverable.
If you are concerned about performance issues due to antivirus being installed on the servers, there are many customisable actions that can be configured in the on-access scanner to exclude certain folders such as ACCESS database. Or you can configure to scan new or modified files on write and not read. If the performance is your concern i would suggest scanning new or modified files. This will limit the amount of processing and allow you to utilize the server better.
I do stress though antivirus should be installed on all your desktops and servers - and in an ideal world every file should be scanned, but as a compromise you could try the above.
I hope this helps and i`m happy to explain further.
av-outsource