Searching nearly free Security-Solution for School-Computer-Network

Definitely consider using OpenDNS for web content filtering and security. The have predefined levels of security and web filtering to make your job even easier. Advanced settings allow you to display custom messages to users when a resource is blocked, view stats and access logs, among tons of other useful features.

OpenDNS is the leading provider of free security and infrastructure services that make the Internet safer through integrated Web content filtering, anti-phishing and DNS. OpenDNS services enable consumers and network administrators to secure their networks from online threats, reduce costs and enforce Internet-use policies. OpenDNS is used today by millions of users and organizations around the world. OpenDNS makes networks in homes, schools and businesses safer, faster, smarter and more reliable through Web content filtering and navigation services.

I have excellent experiences with Faronics DeepFreeze on school computers. Faronics offer special rates to educational facilities, IIRC the costs per machine are something like $15. Anti-Virus and Malware scans are not only time consuming, quite often they do fail and the sysadmin will have to spend a lot of time solving problems, with DeepFreeze these annoyances are a thing of the past, simply restart the computer and it will be in its pristine state. It just works.

Faronics Deep Freeze helps eliminate workstation damage and downtime by making computer configurations indestructible. Once Deep Freeze is installed on a workstation, any changes made to the computer—regardless of whether they are accidental or malicious—are never permanent. Deep Freeze provides immediate immunity from many of the problems that plague computers today—inevitable configuration drift, accidental system misconfiguration, malicious software activity, and incidental system degradation.

Deep Freeze ensures computers are absolutely bulletproof, even when users have full access to system software and settings. Users get to enjoy a pristine and unrestricted computing experience, while IT personnel are freed from tedious helpdesk requests, constant system maintenance, and continuous configuration drift.

Since the client machines are running XP, you can also use Microsoft's Windows SteadyState, a similar product which happens to be free.

Of course, it's always handy to have a 'clean' HDD image for each machine ready to be applied at a moments notice, if push comes to shove, EASEUS ToDo Backup is a free program i can recommend here.

As for safeguarding the network, recommend Vyatta which doesn't have very high system requirements and can be installed on a rather old computer:

The Vyatta Community Edition (VC) is award-winning, Linux-based, open source software providing routing, firewalling, VPN, intrusion prevention, and WAN load balancing services, among others, for your network. When you run Vyatta on a standard x86 hardware system, you'll create a powerful network appliance that can run circles around proprietary systems.

I'd use Microsoft security essentials (its free on individual systems,and fairly light) and untangle for a sitewide security setup- its a gateway server . If its a wired network, you can run untangle on a VM without any changes to your current setup.

Between the two, you should have most things covered. Untangle has a web filter, AV gateway... and a load of other things, and should run on your hardware. MSE dosen't have a very restrictive EULA, and will work as long as you have a legit copy of windows.