What kind of virus scanning software are you looking for? Non-administrator accounts for your users and something like ClamXav to scan e-mail attachments along with a good firewall will usually work just fine.
Do you need central management of all 60 clients? If so, you may need to go with something like McAfee Security. That is what we have at work and it isn't horrible on machine resources. If you can trust your users to install and keep current themselves, then I would second ClamXav. That is what I used when anti-virus was mandatory, but before we were required to use McAfee.