Symptoms

Does Top look like this?
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
9697 root 25 0 77556 67m 2164 R 19.8 3.3 46:16.88 MailScanner
9705 root 25 0 77560 67m 2164 R 19.8 3.3 46:12.57 MailScanner
9712 root 25 0 77560 67m 2164 R 19.8 3.3 46:07.97 MailScanner
9714 root 25 0 77560 67m 2164 R 19.8 3.3 46:06.07 MailScanner
9719 root 25 0 77428 67m 2164 R 19.8 3.3 45:57.36 MailScanner

Solution

Stop the associated services:

service MailScanner stop
service sendmail stop

Sendmail may fail to stop, don’t worry too much.

Confirm you have the RPM forge repo

ls /etc/yum.repos.d/
rpmforge.repo

Install the new method of calling Clam

yum install clamd clamav clamav-db clamav-devel
service clamd start
freshclam

Head to http://kb.parallels.com/en/2997 and download the MailScanner RPM for your version of RH, eg:
cd /usr/src
wget http://download.swsoft.com/ensim/download/pro/linux/kb/2505/i386/rhel4/mailscanner-4.61.7-2.noarch.rpm
rpm -Uvh mailscanner-4.61.7-2.noarch.rpm

Edit MailScanner config to use new method

pico /etc/MailScanner/MailScanner.conf

The line Virus Scanners needs to change to:
Virus Scanners = clamd

Turn it all back on

service MailScanner start
service sendmail start