Steps:
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 , 127.0.0.3 www.sitenameyouwantblocked2.com and that site will no longer be accessable.
-example-
127.0.0.1 localhost
127.0.0.2 www.blockedsite.com
127.0.0.3 www.blockedsite3.com
-->www.blockedsite.com and www.blockedsite3.com is now unaccessable<--
5] Done!
or simply paste this code in run command
notepad c:\WINDOWS\system32\drivers\etc\hoststhen follow step 4
you can unblock them by just deleting the site that you enter
Hope it helps..