Be extra suspicious if (a) the greeting card doesn't address you by name; (b) the card sender's name isn't included in the body of the email or the name isn't familiar; and (c) it's not a holiday, a birthday, or any other occasion that might warrant a card. If the card requires that you install a special viewer or tries to download a file to your system, treat it like a trojan. Cancel the download and scan your system with up to date antivirus software.
Be extra vigilant. If you receive a card from someone you know but you aren't quite sure it's legit, compose a new email to that person and ask if they sent you a card. Don't reply using the email you received - the From address just might be bogus.
In addition, you should always install anti-virus and/or anti-spyware softwares. When you are brought to the "bad" websites that try to install malwares/spywares to your computer, the security softwares will stop them.
Here are the FREE anti-virus and anti-spyware that I am running on my system.
While traditional security companies had gotten relatively good at addressing technical threats like viruses, they were failing to prevent a new breed of "social engineering" tricks like spyware infections, identity theft scams, and sites which send excessive e-mail. To address this challenge, we built a system of automated testers which continually patrol the Web to browse sites, download files, and enter information on sign-up forms. We document all these results and supplement them with feedback from our users, comments from Web site owners, and analysis from our own employees. The software for Internet Explorer and Firefox summarizes website safety results into intuitive red, yellow and green ratings to help Web users stay safe as they search, browse and transact online.