What you can do is create a User account for your kids in windows at first create the account and give it administrator privelages. Then while inside their account open up internet explorer and go to Tools then Internet Options. Click on the security tab, the internet (globe) should be highlighted. Next Click on Custom Level Scroll down to the option that says "enable downloads" Turn it to disabled.
While your there, If you would like you can go over to the last tab which is labeled "Restricted Sites" and you can enter any sites in there that you do no want your child access to.. For example r0tten.com Also if you know of a few sites you dont mind your kids downloading from for example Yahoo.com..you can go the the "trusted sites" option and put the sites in..and they will be able to download from those.
After you have finished with that portion. I recommend going to the next option. Which is the Privacy tab and move the slider up to Medium-High.
Finally go to the Content Tab. And enable the content advisor. Here you can adjust that slider to filter out sites which contain vulgar and adult-themed material.
The rating system is as follows.
Level 0- G
Level 1- PG
Level 2- PG-13
Level 3- R
Level 4- NC-17
Choose whatever rating you think is appropriate when your finished it will prompt you for a password..enter something kinda complex..and close the browser..
Last..Go back to the user accounts and remove the administrative privelages from your kids account and click "limited account" this will prevent them from being able to make any changes to system settings on the computer.
And most important put a password on your account.
Problem should be solved..your kids should be safer. Anyquestions just ask.