Chances are excellent that even with a clean install you basically have adblocking software already installed on your computer. This applies to both Windows and free unices such as Linux or FreeBSD.
All you have to do to make use of this is add one simple line to a text file. The file, in all(?) cases, is called hosts. On a Windows XP system, it should be located at C:/WINDOWS/system32/drivers/etc/hosts and on Linux or FreeBSD at /etc/hosts instead.
First, you need to know an advertiser's domain name. This is pretty simple to accomplish: just configure the preferences in your browser to ask you what to do every time a webpage tries to set a cookie. On Firefox, for example, you would ensure that the Accept cookies from sites chekbox is checked, and the Keep until: dropdown list is set to ask me every time. Then, when it asks you if you want to accept a cookie from a domain like ad.doubleclick.net you will know the name of the enemy. Just make sure you don't do this for the domain of the website you're actually trying to visit, such as http://www.techrepublic.com or sob.apotheon.org.
What you do with that URL (let's continue using ad.doubleclick.net as our example -- it's probably most geeks' favorite advertiser to hate) once you have it is easy. Just add a line like this to your hosts file:
It's that simple. I have a line exactly like that in my own hosts file on this computer, running FreeBSD.
This works because your computer (using something like Windows or FreeBSD) normally uses DNS to resolve hostnames to IP addresses, but if there is an entry for a particular hostname in the hosts file, that takes precedence. 127.0.0.1 is the localhost address -- it just points right back at your own computer. Thus, when a website tries to insert content from an advertiser with the hostname ad.doubleclick.net, your computer will resolve that hostname to the localhost IP address instead of what the DNS servers say it should be.
(note: pretend those forward slashes in the Windows path for the hosts file are backslashes)