Yes MS Security Essentials gets my vote.
As you are only running 8 PCs you can email Kaspersky sales direct and ask if it is OK to purchase home editions.
Perhaps 1 pack of 3 PC and 1 pack of 5
I also run my own IT support company and cleared this with them for small offices a while ago.
Also do you have a server?
I would also look at getting Malware Bytes AntiMalware http://www.malwarebytes.org/mbam.php
This program is the best in malware removal and protection and no person involved in IT security should be without it. Paid version will allow auto updates / schedules etc.
Other FREE programs include:
Spyware Blaster - Browser protection http://www.javacoolsoftware.com/spywareblaster.html
Spyware Guard - Hijack Protection from the same company http://www.javacoolsoftware.com/spywareguard.html
Hosts MVPS - Parasite protection via a Hosts file http://www.mvps.org/winhelp2002/hosts.htm
You could also have a look at some sort of content filtering program such as OpenDNS or K9
Hope that helps and any questions just ask.