Basically, your wallet's hacked. Someone gained access to your private key & they did spend all of the btcs in it. The same case did happen with meh a few days ago. However, the best thing that you can do is to format your computer & start with a new wallet (like meh) & you'll be fine. Simple as that. Take that as a lesson & next time dont download random things over the net.
Just to add to this answer: most likely, you are 100% correct, altough it's usually done by a keylogger, or remote controll software that has infected your pc... not by really hacking the wallet. Just to be clear: if you used a password to protect your wallet.dat and your pc is clean, chances of a wallet getting hacked are very, very slim.
A couple lessons:
- always use a virusscanner, malware scanner and a firewall
- password protect your wallet.dat
- if possible, use a hardware wallet or paper wallet