Greeting card scams arrive in email pretending to be from a friend or family member. Clicking the link to view the card typically leads to a booby-trapped web page that downloads Trojans and other malicious software onto the systems of the unsuspecting.
I received one that seemed to be from my 'number 2' son (it mentioned his first name), whom I've not seen or heard from for some time. When I clicked on the "view card" link my pop-up blocker and anti-virus program went mad! Luckily I pulled the plug (litterally) and managed to clean up the computer (a couple of nasty bits of software did get through as they were not recognised).
To many users an attack like this (yes attack) may well go unnoticed and software they run in the background will register the websites you visit, log your usernames and passwords at login and report back to the perpetrator, who will then be able to access your bank etc. A re-boot will not cure it as the software sets itself up to run at start up.
Best advice I can give is: If you receive an online "Greetings Card" and you don't recognise the sender, it's not your birthday or you are just suspicious of it Delete it.