With regards to your subsidiary question ("How did they get the correct email address"?):
Spammers and fraudsters generally get hold of addresses in two main ways. Firstly, they use software to scan web pages and forums (etc) to collect every address that they can find. Secondly, they simply generate millions of addresses at random, knowing that some of them will be genuine. Either way they're not specifically targeting to you. They just know that if they send out, say, 100 million emails, someone will fall for their trick.
You've also asked about opening an attachment. Ideally, you should never even open an email, yet alone its attachment, unless you're confident about the source. That means that you should always disable the preview pane in any browser (so that the email doesn't open in that pane as soon as you click on it). In Outlook Express the method is to go to View>Layout. Then click to remove the tick alongside 'Show preview pane', click 'Apply' and 'OK'. (Other browsers are usually similar).
Opening an email from an untrusted source can result in generally minor problems to your PC (although if they ultimately lead to you giving away your bank details, through key-logging techniques, they can still be disastrous). Opening an attachment can run a program which will do whatever it's designed to, including installing a virus or even wiping your entire hard drive.
Chris