Their can be many reasons to block websites from your computer such as you want to keep kids from going onto malicious sites or you just want to stop yourself going onto sites, for me I blocked a bunch of shopping websites so I don’t spend any money such as ebuyer.com, play.com and amazon.co.uk.
To block the websites I used something called the host file you will find this in “%SystemRoot%\system32\drivers\etc\” for XP machines for other machines you might need to look for it.
To open the file you can double click on it and open it in notepad you will get examples in it showing you how to use it but some people don’t always understand it.
Once inside it you can type in at the bottom of the page “127.0.0.1 example.com” the 127.0.0.1 is your local machine which you are on right now and example.com is the site that you want to block from your machine, so try it out and you will find that the list slowly gets bigger and bigger.
If you want to edit it often I recommend making a shortcut to the file right click on it and click create shortcut and drag it anywhere you like.