I found this web-site that offers a parental control program. I don't know how well it works but it is worth a shot. Also some browsers and ISP's have parental control settings like AOL, Earthlink, and MSN.
Sorry to disappoint you but no, there's not really such a thing as an 'Adequate' product available. Not if you want your sons 'protected' anyway. Any products available only give an illusion of protection, and the best of them are so restrictive that the internet becomes a virtually useless tool to use as a result of their restrictions.
The best approach, by far, is twofold:
* Educate your sons about the dangers to the computer itself when websurfing the 'seedier' sections of the internet. Such habits bring unwelcome 'spyware' intruders which can make the system unusable. Read through some of the discussions in our Security forum section to see the mess some people have to untangle, and the difficulty of sorting it out! Those problems usually result from clicking on advertisements, downloading 'free' stuff, chasing 'cracks' to get copied software working etc. etc. etc. All those activities which quickly link to or through porn, in other words. It's best to avoid them as far as possible and learn protection techniques when you need to go quickly past them.
Don't be fooled by seeing 'porn' links in the browser history. 95% of all web-browsing will generate those, simply from onscreen advertisements. They don't necessarily mean that someone has been viewing pornography.
* Talk openly and frankly about matters of human relationships in the home with your teenaged sons, so their curiousity isn't heightened by the subject being 'forbidden territory'. Yes, even those more 'shocking' questions they may have! That's the best 'protection' you can ever give them ;)
Your computer won't do it for you. It's not a 'surrogate parent', and even if you put restrictions on it, being teenaged your sons would almost certainly find ways around it anyway. Software won't 'do it' for you, I'm afraid.
You might also care to read this thoughtful and related recent forum discussion:
Catweazle is correct that there is very little that software alone can do. A company that I have been a customer of in the past and am currently partnered with at work (I work for a company that develops websites for churches) is Integrity Online. They offer a variety of services from filtered internet access to free accountability software (tracks and reports access to potentially inappropriate content). The nice thing is that their internet filters aren't mindless programs; rather, they are filters that are actively monitored and maintained by the company.