1. Keep your web browser up to date. This helps to fend off known vulnerabilities. Turn on browser automatic updates and stay away from un-known or un-verified browser ad-ons. I recommend Mozilla Firefox for it's intuitive security controls and large number of verified add-ons.
One add-on in particular I highly recommend is
NoScript.
"this free, open source add-on allows JavaScript, Java, and Flash and other plugins to be executed only by trusted web sites of your choice (e.g. your online bank), and provides the most powerful Anti-XSS protection available in a browser. NoScript's unique white-list based preemptive script blocking approach prevents exploitation of security vulnerabilities (known and even not known yet!) with no loss of functionality..."
2. Use anti-virus software that inspects real time traffic and transfers. I use and recommend Ashampoo's AntiSpyWare 2 Guard. It also protects the registry and gives you full control of how it handles different file types.
3. NEVER click on links contained in suspicious e-mails even if you know the person sending it. Subject line "FW:this is so funny, I found you on this video site" is just one example of a suspicious looking email. When in doubt, open a new browser window and go directly to the website you're trying to see. Never shop from sites that you arrive at from clicking links in marketing emails, also known as SPAM.
4. Use the "Remember My Password" option sparingly when logging into Internet sites. If your PC was to be hacked or stolen, those passwords can be retrieved easily. I recommend using your browser's password storage option for sites that can't affect you financially, politically, or professionally.
5. If you are worried about using credit cards over the Internet, purchase a prepaid card such as a re-loadable Visa or American Express card. They act like credit cards but have a spending limit up to whatever you put on them. You can also call your bank and adjust the daily spending limit on your credit/debit cards.
