Thank you for your advice. Understood, the system itself is clean. The key was compromised.
Of course the key was compromised. If it hasn't been compromised, no transaction could have been made with this private key.
The important question is HOW it got compromised.
Do you arleady know how it happened? Did you enter it on a website? Missing/Lost backup?
If you are not sure about how this happend, it would be careless to assume the system is clean.
A simple, self-coded malware is NOT being recognized by anti virus software. Fooling AV software is way easier than it should be..
So unless you are 100% sure how the key got compromised, i'd suggest to be more carefully when storing cryptos on your PC again.