It is very difficult to trace ,because it is usually attached , inside something else ,where people sending it, don't even know it is in their stuff ,that is being sent , just like you just experienced, where it was sending out emails on your behalf with your name attached to it, by attaching itself to your email address book.
So...
By creating a bogus email address beginning with say AAA@hotmail this usually stops it from getting to the legit entries in the address book, that follow on underneath.
So ...
What will happen if attacked next time.
You will get an automatic email responce saying that AAA@hotmail could not be sent.
This tells you that you now have Malware in your system & you now know what to do to get it out.
Also...
If you run AVG virus protection (free) this will tell you if there is any in your system each time it scans & it will tell you what sites to go to to have it removed.

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Cheers.