To block certain websites on your PC, here' s the trick (for Windows):
Step 1: Click the Start button and select Run. Now copy and paste the following text in that Run box:
Choose "Notepad" to open it.
Step 2: Go to the last line of the file, hit the enter key and type in the following format:
Eg: If you want to block facebook.com and myspace.com
Step 3: Save the modified file.
That's it. None of the above sites can be visited on your computer. One per line if you want to add more web sites. In this way you can only block website one by one, if you want to block websites in large scale, you can use a professional web filter which will help you easily block websites by keywords or URL. I have Aobo Filter(http://goo.gl/YWzOiM) running secretly on my PC. It is not only a web filter but also a Program blocker. It can block games, applications from running on PC.