If you would like to block/unblock a website from being viewed on your Windows computer, follow these steps:
C:\WINDOWS\system32\drivers\etc
First backup thehosts file.
Open hosts file with notepad.
At the end of the line you can see as per below.
127.0.0.1 localhost
Append the following line at the end
127.0.0.1 BlockSitename.com
# replace the BlockSitename.com with the site name that you want to block.
then go to Start->Run-> type ipconfig /flushdns
Note: That will only stop the domain name to go to the website but if you know the IP address of the site from DnsStuff or NetworkTools then you can browse the site.