You're confused about how post and boot work.
At post a snipet of code from the boot sector on the boot drive is read into memory and executed. This code runs the boot loader. If you delete the boot loader, no post. If you modify the boot loader, you are still using the old (Vista/Win7) boot loader, just calling the XP boot loader with it.
Do your OS deletion and run an XP repair install. That will overwrite the code on the boot sector of the boot drive and it will then call the XP boot loader (boot.ini).
At post a snipet of code from the boot sector on the boot drive is read into memory and executed. This code runs the boot loader. If you delete the boot loader, no post. If you modify the boot loader, you are still using the old (Vista/Win7) boot loader, just calling the XP boot loader with it.
Do your OS deletion and run an XP repair install. That will overwrite the code on the boot sector of the boot drive and it will then call the XP boot loader (boot.ini).