The location of the hosts file depends on your operation system.
Windows XP, Vista and Windows 7 C:\WINDOWS\system32\drivers\etc\hosts
Note! Make a backup of the hosts file and put it somewhere safe, in case you mess up.
1. Open the hosts file using Notepad or other plain text editor.
2. Find the line 127.0.0.1 localhost
3. To block, for example, the website YouTube, add this text under 127.0.0.1 localhost: 127.0.0.1 YouTube
4. Save and close the Hosts file.
5. Reboot your computer to apply the changes.
6. Try to open YouTube (for example) into a browser.