In Windows XP, the hosts file is found here:
C:\Windows\System32\Drivers\etc\hosts
Once you have found the hosts file, right click it and select Properties. Uncheck the "Read-Only" attribute (if checked). Click OK.
Double click the 'Hosts' file and select "Notepad" from the list of programs to open it with.
Once you have the hosts file open in Notepad, you will see an entry such as:
127.0.0.1 localhost
To block a certain web page from loading, add the following:
127.0.0.1 www.webpage.com
Your Hosts file will now look like this:
127.0.0.1 localhost
127.0.0.1 www.webpage.com
Add as many web addresses as you like in the same fashion.
Once you are finished, go up to 'File' and click 'Save'.
The web pages listed in your 'Hosts' file should be blocked from access. It may be necessary for you to reboot your computer for these changes to take effect.