Hi Yezinki, and welcome to the forum.
I believe a possible cause is that the detection could have been a false positive, which was later corrected, so that following the update is was no longer detected. Since the file seems to have somehow re0created itself, it is either a system file with the ability to do that, or it is indeed malware. Can you post the full name and path of the file detected, and if you remember, the name of the trojan as described by Avast, please?
In general terms deletion is never a good first option, it's always better to quarantine, or even to ignore while further investigation is carried out, in case it is a false positive. The file concerned can then be examined, "Googled", uploaded to an online scanner service etc for checking.
It's always a good idea to update any database of security software before a scan.
To run a boot-scan, start Avast, (Right click the tray icon, select "start Avast...") it will take half a minute for the GUI to load, select "menu", then halfway down the list, "schedule boot time scan", and follow the prompts.
Generally a boot scan is only indicated if you have an infection that is proving difficult to remove. Otherwise a normal scan without archives is usually adequate.
In your case further investigation of the file involved is recommended.
[Edit] PS, thanks, but I'm not sure I qualify as a "smart genius".
(although some parts of me have been described as "smart" before, the word "genius" has never been involved in those compliments. Other words....)