No operating system, not even Qubes, can help you if you're installing it on hardware that is already compromised.
This includes CPUs, GPUs, SSDs, HDDs, the motherboard, BIOS/EFI/UEFI, and all relevant firmware.
Unfortunately, in today's world of undetectable supply chain attacks, there are no easy solutions.
(Tools like [Anti Evil Maid (AEM)][AEM] can help with *maintaining* the trustworthiness of your hardware, but not with establishing it in the first place.)
Some users have chosen to use tools like [Coreboot], [Heads], and [Skulls].
You should [verify] the PGP signature on your Qubes ISO before you install from it.
However, if the machine on which you attempt the verification process is already compromised, it could falsely claim that a malicious ISO has a good signature.
Therefore, in order to be certain that your Qubes ISO is trustworthy, you require a trustworthy machine.
But how can you be certain *that* machine is trustworthy?
Only by using another trusted machine, and so forth.
This is a [classic problem].
While various [solutions] have been proposed, the point is that each user must ultimately make a choice about whether to trust that a file is non-malicious.
(Option 3 violates the Qubes security model since it entails transferring an untrusted ISO to dom0 in order to burn it to disc, which leaves only the other two options.)
Considering the pros and cons of each, perhaps a USB drive with non-rewritable (or at least cryptographically-signed) firmware and a physical write-protect switch might be the best option.
