Reimaging a system generally eliminates any remote access tools, but if the ACPI Windows Platform Binary Table (an executable that lives in PC firmware by design that recent versions of Windows loads and runs automatically without ability to disable) has been compromised through a malicious firmware update to automatically install a RAT on power on, you may need to go so far as to manually flash the chip holding the firmware to a known non-compromised version. This is a highly technical operation and not for everyone.

Not using ACPI-based Windows-intended hardware, which will unfortunately consist of most of the PC-based motherboards you can purchase on the market today, can help avoid this situation in the first place.