Sort out your file type confusions with the Newbie Club files tutorial . The Bits and Bytes tutorial will clear up questions on file sizes, and how they affect your downloads and disk storage. PC Guide is a great place to start to learn more about your PC and how it works. It also has a forum where you can ask for help or information, and troubleshooting guides for problems. This site covers all the major email programs for both Mac and PCs.
Easy Internet has all the basics, including a web glossary.
Also forums and tutorials. I used this when I began on the web. It's a great resource, clearly written, starting with the very basics. Accepting credit cards on-line explains what it takes, and the safety aspects. WinAsk is a message board dedicated to Windows problems.