Make sure you have a security policy in place -- The security policy is the formal statement of rules on how security will be implemented in your organization. A security policy should define the level of security and the roles and responsibilities of users, administrators and managers.

Make sure all of your operating systems and applications are patched with the latest service packs and hotfixes -- Keeping your systems patched will close vulnerabilities that can be exploited by hackers.

Keep an inventory of your network devices -- Develop and maintain a list of all hardware/software components, and understand which default software installations provide weak security configurations.

Scan TCP/UDP services -- Turn off or remove unnecessary services. Unneeded services can be the entry point attackers use to gain control of your system.

Establish a strong password policy -- Weak passwords could mean a compromised user account.

Don't trust code from non-trusted sources.

Block certain e-mail attachment types -- This list includes .bas, .bat, .exe and .vbs.

Don't provide more rights to system resources than necessary -- Implement the concept of "least privilege".

Perform your own network security testing -- Find the holes before the attackers do!

Implement "defense-in-depth" -- Don't rely on just one control or system to provide all the security you need.