Suppose you wanna block www.orkut.com then
or some other site like the below one

1] Browse C:\WINDOWS\system32\drivers\etc

2] Find the file named HOSTS and open it with notepad

3] Go to the last line - "127.0.0.1 localhost" - under that add
127.0.0.2 www.orkut.com, and
then that site will no longer be accessible !!!

********************************************

C:\WINDOWS\system32\drivers\etc\hosts

127.0.0.1 localhost
127.0.0.2 www.orkut.com

********************************************

If you wanna block more sites you can add more lines to this by addling
1 to the Loop back IP

********************************************

C:\WINDOWS\system32\drivers\etc\hosts

127.0.0.1 localhost
127.0.0.2 www.orkut.com
127.0.0.3 www.yahoomail.com
127.0.0.4 www.gmail.com

********************************************

Group Policy
Open Group Policy (Run -> gpedit.msc)

Go to User configuration -> Windows settings -> Internet
explorer maintenance->
security -> double click Security zones

Enable security zones and privacy by selecting import the
current security zones and privacy settings -> Click on modify
settings -> Click on privacy -> Click site -> In the address
of the web site tab add the site name you wanna block -> Click block
-> click OK -> OK -> OK


This will block the site.........