There really isn't a suitable software-only solution, you should employ a hardware network/internet interface, such as
SonicWall - there are others, this one I'm familiar with - look at their T50 and T70 series of SOHO appliances; fine for small networks - wired or wireless). Essentially, it amounts to a front-door solution, providing firewall, antivirus, antitrojan, antispam, and content filtering - as administrator, you pretty much can restrict network access to anything you don't want the network's computers to reach. Before implementing any such solution, you also should be certain your network's machines are absolutely clean, fully updated, and are configured for safe internet use - for instance, browser and chat and email clients should be granted only necessary privileges, and users should not be able to operate the machines via an administrative-level account (IMO, a real big flaw with Windows systems). I would guess you could get to where you wanna be - a network you control and don't hafta worry about - for well under $1000.
Internal-to-the-network firewall, antivirus, antispyware etc software, resident on each machine is a good idea, providing a second layer of protection.
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