This was a "Trojan Horse" attack in my opinion. An email is sent from a know good source (who has also been infected in the same manner) with an innocuous attachment. Normally either patriotic, religious, or "friendship" oriented to ensure that the receiver opens it. The attachment will have a recognizable extension (like jpg or wmv) but that is a mask for an executable code string that runs on your command.
Once it runs; its first task is to hide itself from your antivirus program. Its next task is to use your contact list to mail itself to everyone you know.
The only sure way to defend yourself is to regularly scan your computer and files with at least two different malware programs. You will need an antivirus and a malware detection program. Think of an antivirus as a vaccination to prevent an infection and a anti-malware program as an Antibiotic to kill the bug that bit you.
While I use AVG Free to protect myself from viruses, AVG will not stop a Trojan. AVG Free will scan for and clean "root kit infections." It is the only free antivirus that does that that I am aware of.
Malware (or trojan) is "invited in" by the user. They need to be screened for and destroyed regularly to keep your system fast and bug free. I use two weekly to keep my systems running and avoid infecting my friends.
Malwarebytes from Malwarebytes
is free and does a fine job.
Hitman Pro 3.5 (google the web site) is also free to install and scan. It will also give you a free "cleaning" if you do get hit. After the first free cleaning you will need to buy it for further infections (it will still scan though).
Firewalls are very tricky and often are installed improperly. A bad firewall installation will make your computer nearly unusable, and you will soon tire of the hastle and it will be deleted. Windows comes with a fair firewall that works well enough for most users. I would leave it do its job as defined by Microsoft and know that you can still print to a shared printer on your home network.