OK... It took me a few minutes of research to find out what happened exactly. So this guy installed a trojan from this site:
http://electrum-wallet.com/instead of installing the electrum wallet from this website:
https://electrum.org/A very very costly mistake instead. Also, just a few days back, Theymos had instructed us not to transfer the BTCs in very large blocks. He ignored that warning also.
Note that sending all of your BTC in a single transaction is likely to be harmful to your privacy; if you care about this, you might want to send it in several smaller transactions to several different addresses on the new wallet.