Okay someone stole BTC from me today. No idea how. I am quite surprised that it happened. My wallet was unencrypted and was created in 2011.
The thief's transaction did not have any fees so was not picked up when I came to know about the theft.
Thereafter I copied an old wallet.dat and replaced it and created a new transaction with a fee. I want to stop the thief from getting my btc.
http://blockchain.info/double-spends