No, i don’t have any DVD Drive, the hard is new so no Windows was installed in the past, but in the broken one was Windows 8.1 so i guess it should work.I don’t have any machines left to try another program (i tried 2 sticks with different burn programs with same result).Now it doesn’t want to start no more(video is not working, when i press the power on button it just starts the LED’s for like 10 sec and the fans work at normal speed for 10 sec then all the fans starts to spin at max speed, i left him on for like a hour but it wont start, it just spins over and over without any progress) Guess i’ll just leave as it is and give up..does it have a DVD drive? I found this question asking same thing and one person fixed it by making an installer on DVD instead - https://h30434.www3.hp.com/t5/Desktop-Boot-and-Lockup/HP-DC7800-refuses-to-boot-from-USB/td-p/6412913
do you have newest bios? this guy fixed it by updating bios - https://www.bleepingcomputer.com/forums/t/579505/at-wits-end-hp-dc7800-sff-will-not-boot-from-usb/
did you set up rufus for legacy boot?
i found people having problems installing windows vista on it in 2008 via USB...
Which makes me wonder, was win 10 an upgrade from win 7? Cause having USB be seen by bios is one thing, having win 10 recognise a PC from 2008 is another. It may not even have drivers for the chipset on PC. The installer may not run... and there is the problem of drivers for the hardware.
Do you have a win 7 installer? I would suggest installing it, updating it and then upgrading it to 10 as then you bypass fact windows won't have any drivers for hardware as it uses Windows 7 ones.