What's a Hosts File?
Technically, it's a file on your computer that is checked before any request to the Internet is made. If there is an entry for the website you're
requesting, Windows will go to wherever the file says, rather than where a DNS server out on the Internet says.

Sounds geeky. Why the hell do I care?
There are groups of good hearted surfers out there that spend some of their free time compiling lists of websites that dish out viruses, spyware,
advertisements and other things you'd rather not have on your computer. When this list takes the form of a hosts file, something great happens:
your computer quits loading all this malicious crap and you can stop calling me for help. It sounds almost too good to be true.

Great. Any possible side affects?
Like all good things, yes, there are some possible side affects. The only one that I can think of was from a friend of mine whose currently in an
Internet Marketing class. She actually needs to be able to view ad's on the web. If you're like her, steer clear of this until your situation changes.

Bill, I trust you with my children. Let's do this Hosts thing already.
This hosts file we need to obtain can be found on the web. It needs to be copied to a location deep inside of the Windows system directory structure.
Once there, it needs to be updated regularly as there are new threats everyday. Sounds like a pain in the ass, right? Well, it would be but someone
wrote a slick little program that does it all for you. You'll want to download this file and save it to your desktop or somewhere where you can find it
Next, run the program. If you get some error about not having .NET installed, download it from here first.
Once you're in the install, all the default options are just fine. Once the program is running, you'll want to make the following selections:
-Click "Options" near the top left.
-Place check marks in the following boxes: "Start With Windows" "Always Start Minimized" and "Auto-Update On Start"
-Only do this if you rarely shutdown / reboot your computer - Check "Auto-Update at" and select a time when you're usually not at your computer.

That's it?
Yepp, pretty much. Now you'll see a yellow padlock icon next to the time clock in Windows. That's the program running. If you believe that for some
reason the Hosts file is screwing things up, double click the lock then click "Rename HOSTS (Activate/De-Activate). That'll stop it temporarily.
You can re-activate it the same way or you can just uninstall the program at that point.


Enjoy.
Bill

Just in case you actually want to read the boring details on this:
More on Host Files.
More on the Host File updater.