Processes, are not able to start at boot-time as the "processes" is a OS based concept. The closest method you can do is replace the bootloader of Windows with your bootloader then load the Windows through your bootloader, and before loading the OS your
Bootloader can perform any calculations or start files up as soon as Windows loads. This is the only way to start processes closest to Boot time.
In windows vista+ OS this technique would not work because Windows Vista+ has Session separation which by design separates processes into sessions and if you were to start it as soon as the OS starts the process would crash the entire OS as the process would
not be able to fit in any session as it started before "Winlogon.exe"
process. Therefore it is not really possible.