I think there are  some settings in IE and Windows to Block unwanted Porn sites but these settings might not help you fully .I recommend to use Softwares to Block those porn sites.

Softwares

1.Net Nanny

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You can download these software and you can protect your kids form accessing unwanted stuff.

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2. NetDog Porn Filter

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Using NetDog Porn Filter you can block Porn Stuff also.

3.PC Tattletale Parental Control and Monitoring   Software.

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Using the above software you can restrict access to unwanted sites and monitor your kids from Browsing

Parental Controls in Vista

One of the finest features that Windows Vista have ever been implemented  is inbuilt  Parental controls which is  not in Windows XP.

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So it’s a good news to people who are using vista doesn’t need any software.So Vista has been in a way where security has been improved as Compared to Windows XP SP2.Read more

 

Manual Blocking

May be this may not do the job for you because you can not add all those porn sites to the hosts file. Read More.