If you have a decent router you should be able to use that from within its control panel, I have a netgear max and am able to do loads of tricks.
I have 3 PC's in the house, my netbook, my desktop, and the main house desktop.
In the control panel I'm able to restrict my network to an ip range of only 3 ip's, 192.168.0.1 / 192.168.0.2 / 192.168.0.3
Each PC is assigned a reserved ip.
Once this is done I can restrict the internet access to each ip individually, you can either do a keyword block, simply enter the keyword in the box 'sex', select which ip's to restrict, once done the pc wont be able to connect to websites with the word sex in the address. You can have an endless list of keywords, so you can block a mulitutude of sites. You can also block specific websites, simply enter the url's in a list.
Its the best way I have found of having some decent rliable netwrok control without paying for other software.