Hello, The other day I checked my BitcoinQT software, because I wanted to check my balance, only to find that an unauthorized transaction had taken place for basically all the bitcoin I own... I had my wallet encrypted and everything. I have come to realize that someone somehow stole my BTC, I decided to come to this site to see if anyone could shed some light as to how my wallet was hacked. My BTC address is: 1LW3QuLxkyp92HGvMFio4eYY6puxmnGSDa and the thieves address is: 133tgZXxeJGnoWc467jStKTvj8PB1Hs9kS and the transaction ID is: f1223be531a8120c87c6d3696efd28c58f6e69964ae451957fd9c35391935135.
They stole all of my BTC that I had been slowly mining and saving up. It's a small amount, 2.84 BTC, but it meant a lot to me to have at least some bitcoin. Now I have none.... It is disheartening the amount of malicious attacks that happen within the Bitcoin network, people are cold.
Anyway, is anyone able to give me advice as to how this was stolen from me?

If I buy more bitcoin I don't want it to be stolen again, so I need to close the backdoor I left open for the thieve. It's scary to think that he got a hold of my encryption key, because It was intricate and would be difficult to just brute force, which leads me to believe that my keystrokes were being logged.
Once again, any help would be appreciated. Thanks ahead of time.