In answer to the “blocking off sites” question, I can recommend a free option – OpenDNS.
It is a free DNS service that has some extra bells and whistles. One of those is site blocking. They allow you to block traffic by category and/or by listing specific sites to block (I’d recommend using both – the categories are good but if you find out that a specific site is not categorized properly you can add it yourself).
You can set up your entire network to route traffic through OpenDNS, or you can do it on each individual PC. If you are the only one with network admin rights, no one can get around this. If you are the only one with admin rights on your computer, no one should be able to change this setting when logged in under a different name (XP or newer).
FYI: For those who don’t know what DNS service is, and someone is sure to ask, it is basically the service that translates what you type in your address bar (e.g. http://www.theyeshivaworld.com/coffeeroom) into an IP address (such as 69.123.343.12).