so this all happened due to a trojan? 
I think, yes.
I doubled check my antivir now and I really have installed AVG Antivir AND Microsoft Essentials, but both were not enough to prevent the thief. However, a wallet without password protection was really dumb, life learns you the hard way, even if in that case it was not very hard at least.
From #3 and #1 I think you got infected while using IE, so a drive by infection most likely. Those few minutes while you are out there to get a safe browser... Oh well. Whats buddha.dll though? A quick research suggests sleeping dogs or Call of Duty Black Ops 2. I didnt dig to deep, but I suspect a pirated version/cheat/etc. I wouldnt know why an official release would require a download via 2shared. So #1 and #3 might have followed after you got the dll somewhere insecure. Again, I dont judge, I dont require an answer, just think about where you might have gotten the file.
Aaanyway seems like you are taking a safer route for now. While I usually argue that your own machine is safer than some server on the internet, this depends on what you are doing with your machine.
Bleib wachsam
Hm, I never played neither sleeping dogs nor COD2 on my computer. The thing is that I have received this copy of the OS from the university via MSDNAA and received a copy on my D:\ drive via an usb disk from one of my colleagues since he already downloaded it. So I just needed a new key from the university partner program with MS. Can it be that buddha.dll replicated it to my disk without somebody (antivir, etc.) knowing it? I remember that sometimes my colleagues played COD2 at the university in there spare time. I usually play LOL which is free to play.
I cannot explain it in a different way, because otherwise buddha.dll should be in a folder named "D:\CallOfDuty\buddha.dll" or something?!? Just as a guess.
Nevertheless, I learned my lesson and how "easy" BTC can be stolen from your wallet even if it was reckless to have a wallet without a password, since bitcoin-qt does not enforce it (!!) as for instance multibit does.