1] Browse C:\WINDOWS\system32\drivers\etc
2] Find the file named "HOSTS"
3] Open it in notepad
4] Under "127.0.0.1 localhost" Add 127.0.0.2 www.sitenameyouwantblocked.com , and that site will no longer be accessable.
5] Done!

-So-

127.0.0.1 localhost
127.0.0.2 www.blockedsite.com

-->www.blockedsite.com is now unaccessable<--

For every site after that you want to add, just add "1" to the last number in the internal ip (127.0.0.2) and then the addy like before.

IE: 127.0.0.3 www.blablabla.com
127.0.0.4 www.blablabla.com
127.0.0.5 www.blablabla.com
Enjoy!!!!