Encrypted files by AVG after Reinstaaling Win10

Suat, In future please refrain from carrying out multiple topic postings on the same subject! You won't obtain an answer to your query any faster!. Also please try to keep the whole conversation in one question/thread to speed up any resolution. Please see your other posting (https://support.avg.com/answers?id=906b0000000DsXbAAK).
AVG Guru

Hi all,
unfortunately the AVG Support Germany referred me to Microsoft without a reliable explanation, I need your expertise now.
I had have a AVG Internet Security on my PC (Win10) till I had a virus attack, which led to lose all my data on the ext. hard disc and destabilisation of the system. Then I had to reinstall Win10. Before reinstalling  I had to back up all my data on my PC  to an other ext. hard disc. Now I have some of old files/documents of mine encrypted (still) by AVG Internet Security. Somehow I donot have the needed authorization to open those file, since - I guess- the name of the old PC/System is different than the one now. AVG Support Germany says that, I had to apply Microsoft to get an approval/authorisation 'cause it has nothing to do with the settings of AVG IS, which did not convinced me.
The QUESTION is: how I can open those AVG- encrypted files, which in fact belong to me?
I would really appreciate your assistance. Best Regards. Ayaz
Suat, In future please refrain from carrying out multiple topic postings on the same subject! You won't obtain an answer to your query any faster!. Also please try to keep the whole conversation in one question/thread to speed up any resolution. Please see your other posting (https://support.avg.com/answers?id=906b0000000DsXbAAK).
AVG Guru