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Could my PC be infected? I thought so, but the scans showed no results..
Possible, hard to tell. The only way to be sure to be free of infections is to wipe everything and to reinstall from an installation medium you know is clean and secure.
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Has someone stole my FTP password? I thought this one too, but I changed it..
Yes, changing the password is a good measure.
There's a third possibility how your site could have become compromised: Your hosting provider itself is insecure, e.g. due to outdated software on their server.