Windows XP HOSTS File

To avoid web tracking and web ads, it is often useful to blacklist domains in the HOSTS file.  On Windows XP, the HOSTS file is drive:\WINDOWS\system32\drivers\etc.

Make a backup before editing the file.  To avoid breaking the earlier entries, add new entries to the bottom of the file. 
An entry to black hole a domain will look like the following. 
127.0.0.1       cdn.optmd.com

This shows before and after for a host lookup of cdn.optmd.com.  In the second ping, it resolves to the localhost.