First, download and install the latest version of Firefox. Click that big green button. Save the file and run it to install Firefox.
Once you have the latest Firefox installed, get the AdBlock extension. Just click "Install Now", it's down the page a little.
Now get the latest list of sites to block. Several files will be listed on this site. Right-click the most recent adblock list (e.g. 2006-03-08a.txt) and select "Save Link As"
Save it to My Documents or anywhere else.
Now, to load this list into Firefox just follow these steps:
In Firefox, on the menu, select Tools|Extensions
Click Adblock, then click the Options button
Click Adblock Options, then Import Filters
Select the file you just downloaded and saved, and click Open
Click the Append button
Enjoy ad-free browsing! If you do by chance run across an add, you can block it by right-clicking it and selecting "Block Images from ..."