The HOSTS file can be used to tell Windows the IP address of various network or Internet resources, so improving performance or allowing you to block some sites or resources entirely.
Windows doesn’t make it particularly easy to access HOSTS, unfortunately, but the BlueLife Hosts Editor probably provides everything you’ll need. Just launch the program to view your current HOSTS file, then add or remove entries to suit your needs, and save the results when you’re finished: easy.
Beware, though, if you try to block a site you’ve just visited then it’ll appear not to work, as Windows will cache the previous IP address. Save the new file, click “Flush DNS” and everything should now work just as you expect.