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I'm not sure about my system so I want to completely wipe my HDD and reinstall Windows 10. But I realized that an infected system can also infect a USB bootable drive. Unfortunately, I don't have access to a trusted system to create the USB bootable drive. I scanned my system with a couple of antivirus software (Malwarebytes, Windows Defender offline, Kaspersky, etc.) but still, I cannot be sure because of sophisticated malware, like rootkits.
Any suggestions?
There is no foolproof way. Since you can't trust the system, you can't reliably verify the integrity of the image you will be using to create the bootable USB or, for that matter, of anything else. No matter what you do, it is possible for the malware to have interfered with it.
You could, however, do something the malware would likely not expect. Like make a bootable USB for a lightweight Linux distro, boot live from it, and then make the Windows 10 bootable USB from the live Linux OS. It is quite unlikely for a Windows malware to infect Linux as well, but for that matter, it is unlikely for malware to infect bootable USBs in the first place.
It's very unlikely that malware would be able to run on Windows and Linux, given that executable files for them are different, so it would need to be explicitly designed to infect Linux. For viruses in the boot sector, it'd be harder to stop. I wouldn't particularly worry about a virus in the BIOS, because the BIOS/UEFI is typically on a ROM, meaning it can't be changed, or an EEPROM, meaning you can change it, but you have to utterly erase it first.
Try making a bootable Live Linux CD-ROM and booting into that. No virus can change the contents of a CD-ROM once it's been burned, because it's a ROM. Then, flash the Windows Install USB from the Live CD and then install from the almost certainly safe USB.
