problems with KAV for FreeBSD 6.0

[Roldan Vallejo Olivera]

hello list:
I have a FreeBSD 6.0 running in an HP Proliant GL 370, Dual Xeon 3.2 GHz, 
1GB RAM, and RAID-5 75 GB, in this system we have a BIND DNS service and a 
Sendmail as a mail relay-only server, processing an average of 3GB messages 
daily, we have purchased a KAV license for 4 GB traffic daily, but we are 
having problems: sometimes our server runs out of resources and issues the 
following message error:
"maxproc limit exceeded by uid 0, please see tuning(7) and login.conf(5) ns1 
sendmail[545] syserr (root): <mailbox at mydomain.com>   openmailer 
(smtpscanner): cannot fork: resource temporarily unavailable no queue: 
syserr(root): daemon: cannot fork"

at this moment when I try to login at the server i receive this error:
"login: login: fork: resource temporarily unavailable"

the server replies at ping command, but doesn't allow telnet neither ssh, 
and stops processing messages. we have noticed  this scenario runs out of 
free memory as we read the output of top command:

"last pid: 26888;  load averages:  1.02,  0.91,  0.54  up 3+19:06:18 
03:10:00
615 processes: 4 starting, 2 running, 608 sleeping, 1 lock

Mem: 698M Active, 58M Inact, 188M Wired, 48M Cache, 111M Buf, 3532K Free
Swap: 4096M Total, 24M Used, 4071M Free"

As soon as I reboot the system everything works fine for the next 3 or 4 
days... I don't know what's exactly the problem and of course, I have no 
idea of the solution, can anybody help me, please?
thanks in advance
roldan