ImperoSoftware is always my first port of call for things like this, but as you say you have no client/server network you would be struggling. 20 odd computers in the same building with no server, I think you're missing out mate to be honest.
If you want any information on the level of security, features, ease of management that a server could bring your way don't hesitate to gimme a ring.
*Internet usage policy can be used to make aware the security responsibilities employees have within your business, if they don't realise now, a well worded policy could be the boot up the bum you may want*