I assume you are refering to your HOSTS file? ?
A good description of it can be found in the help file for SpyBot S&D As follows :-
"The hosts file could be described as an address book. While the normal user is used to access other computers on the internet using names (for example security.kolla.de), every computer is accessed by a numeric address at a lower level. You may already have seen this numeric addresses; they look like 127.0.0.1 for example.
Every time you try to access another computer by using his name, your computer looks up his address in an address book. First it looks into a local address book (the hosts file), and only if it can't find the address there it looks in a very big address book in the internet.
So if you want to block an internet website, you could simply redirect this sites name to a place where nothing will be delivered from. Such a place would be your computer for example. The address I already mentioned, 127.0.0.1 is an address that will always point to the local - your - computer. By adding an entry to the hosts file (your local address book) that redirects an ad site to your machine, you would trick your internet browser to think that ad site would be on your machine, and as your machine doesn't deliver ads, it wouldn'get the ad and it will not be displayed."
You can use S&D to ammend your HOSTS file with all the known ad servers and other naff places on the internet (like ads.doubleclick.net etc ) and direct them to your loop-back address (127.0.0.1) Hey Presto - no more ads from them! ! !
S&D will also make the file "Read Only" so it cannot be tampered with. . . It is useful. Youcan edit it to add new undesirables as and wen you like . . (Mine is currently 304K in size! !)
Don't delete it, use it wisely . . .
Well done and Good Luck . . .
:o)