Dr.Web and CentOS5 does not like each other by default.
The version of Dr.Web used in Plesk 8.2.x, is linked against older versions of libstdc++ and openssl.
To get Dr.Web to run on CentOS5, you need to install the some compatibility libraries.
The following should get you sorted:
yum install compat-libstdc++-33.i386 openssl097a.i386