When you use a virtual server (or a old hardware), you sometimes only have 256MB of ram. This is no problem for ISP-config, but only if you disable Virus-Protection for the email-accounts with much traffic. Your server will get to swap as he can with virus-protecion on and much emails.
Instead of disabling antivirus it might be an option to use clamd instaed of clamscan as it uses less resources. You will find several thraeds here in the forum that explain how to switch to clamd.
