hello povobantu and welcome to the Syschat Forums
when you have your computer technician friend with you,
install the program "EasyBCD"
Download EasyBCD 2.0.2 - NeoSmart Technologies
then you both can check the setup of your dual boot, fix it and make adjustments using EasyBCD.
if possible, you can try doing the same method of installation on a different test computer, and then run EasyBCD on that machine.