In no particular order, here are our top tips for keeping your business IT systems secure:

• Use a good anti-virus / anti-spyware system with centralised management.  This doesn’t mean AVG Free Edition which is not licensed for commercial use.
• Use an email security and anti-spam system to prevent nasties arriving through your email system.  Disregard unsolicited commercial email messages (aka SPAM) that still make it through and do not click any buttons or links contained in such messages.
• Keep operating systems and software updated with the latest patches.  At the most basic level this means running Windows Update regularly on each computer.
• Use strong passwords to prevent unauthorised access. You should ideally use a mixture of uppercase and lowercase letters together with numbers and non-alphanumeric characters.  The longer the password the better and don’t keep it written down on a notepad next to your PC.
• Click No to any message asking for authorisation to download and install
  software (unless you are certain that you can trust both the creator of
  the software they are downloading and the Web site source from where you are downloading the software).
• Configure web browser security settings to block unauthorised installations via the web.
• Configure Outlook security settings to prevent automatic downloading of external content and block links to dangerous sites. 
• Prohibit the use of peer-to-peer file-sharing services on your network.  Threats may be disguised as other types of file which users may want to download.
• Restrict use of administrative rights on computers.  Malware gets the same rights as whoever is logged on when it runs.  Running without admin rights limits the damage that can be done.
• Ensure that you take regular backups of everything and test that you can restore the data if necessary.
• Use a network level firewall to control Internet access and ensure your employees don’t access dangerous or inappropriate websites.