Locking down the Windows computers is relatively simple but perhaps not so easy... you need to create and deploy Windows group policy, which would require you to create a Windows domain on the network. Learn it, figure it out, do it.
The Web content is a different approach. The most common solution is a content-filtering proxy server such as WebSense or an education-specific solution such as cymphonix. The simplest approach is a proxy server appliance...WebSense makes lots of different models that do that.
You configure the network so that the only way to the Internet is through the proxy server, and the proxy server both filters and logs the Web traffic.
There are several open-source content filtering solutions such as Untangle or SafeSquid, however these are more difficult to deploy.
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