As I understand it there are a few ways 'hackers' take accounts.

1. Compromised data bases from older games. They take the registered passwords and email addresses and try to access them through verifications like 'forgot player name'. Some people use the same, or similiar, password for just about everything. Once they have this they have access to your email which in turn gives them access to your characters.

2. Key logger. There are plenty of ways one of these nasty programs can get lodged into your computer. However most anti virus software will indeed pick it up if it's up to date. AVG, a free anti virus program, will pick up 95% of threats like these when up to date.

3. You visitd a website that sells Gil, and/or have submitted account information. Sometimes it can be a simple questionaire that asks you for your email information and player name. From there some pass cracks can be used but if you have a password that's decent these usually don't work. Never go to these sites, never submit information about your email or accounts unless you've verified the source. This, I hate to say, is the primary cause of account hacking. Players wanting an easy way out make a deal with the devil and get burned. Also to note that this goes back to my first reason, as once they have any information on you they will have it always until you change it. These are the same people from older/different games. So if you've made that mistake in the past, even visited a website for it, change your passwords.

There are other ways of course, but these are the 3 most commonly associated with account 'hacking'. I find more often than not people fall into one of these categories rather they want to admit it or not. So as the other informed people here have suggested, change your account information and passwords to strong passwords and never give out that information. Also investigate the log out error that some people had since this could also be that. Good luck.