I have about 30 computers in my network.
I have Symantec Corporation Endpoint 11.
I used Symantec since 4 years ago, when it was still version 8.
Finally, I had to upgrade to version 11, when a virus attact disable the realtime and schedule scanning system. That featured did not work at all.
Now, I had almost the same problem, in the realtime system. It become uncontrolable, running continually nonstop. I stop it by uninstalling.
When I reinstall, it come with the same problem.
It also disactivate the antivirus system, so the computer was full with trojan etc.
I think I have to give up the Symantec. We pay so much, but too complicated.
In the meantime, I use AVG free, but it does not have realtime protection.
I would like to ask your opinion, which antivirus is "more or less" the best?
All of the client computers are for internet.
We have Windows Server 2003 and Windows Exchange Server 2003

first of all, get a proper firewall and a front-end for exchange. with a good stateful-inspection enabled firewall like checkpoint NG1, most viruses will be stopped before they have a chance to attack your system.
A spam/virus filtering front end for Exchange is the best way of not receiving mail attached viruses (I'd use Postfix+ClamAV+Spamassassin+Tarpit)

Now in my experience, for a corporate AV Trend Micro is definitely one of the best choices, since Symantec, McAffee and KAV are not just horrible at virus detection, but they also very heavily utilize workstation resources.

Anyhow, removing symantec is a bit of a hassle, you will probably need the removal software supplied by Symantec on their website. But since the workstations are currently infected, I'd say backup the data, and reformat the whole lot of them - 30 PCs can be easily installed in half a day.

Yeah you definetely need a firewall and not the sotware you need like a cisco firewall box, then a program to manage mails we use mail marshal, can configure to block any and all attachment and blank subject line, even if the mail are sent internal very configurable.

but i don't know your budget so you would have to do research and find a cheap efficient alternative

cisco firewall box is not better than checkpoint SPLAT. and it's not as configurable. and it's more expensive

i was not referring to the configurations of the cisco box i was talking about Mail marshal....and i did state i don't know the person budget so they would have to do research...

I remember Mail Marshal, it was replaced by Debian 3.1+Qmail+SA and the spam levels dropped by 92%

so far, I use Microsoft ISA for the firewall.
Another question, are skype or similar programs contribute the risk of having viruses?

in spite of the many rumours, nobody could prove the danger in skype as something tangible