***MOST IMPORTANT!!!***
Have no idea if youtube guy has advised you but what youre doing is pretty risky, do you have ANY important data on the laptop, pictures etc that arent backed up? If yes, you might wanna take the HDD out first and back it up on a pc using a sata-usb adapter.
t31 has covered this, but presuming you made a new partition, formatted that whilst in windows, made it primary (you changed the boot order?) then restarted yeah could cause issues, cos its trying to boot to hdd with nothing as far as windows is concerned.
you should be able to install windows to the new partition though, bootmgr missing shoudnt come up if youre trying to install windows??? so not sure what youve done with the partition. New partition was made in windows? Tell us how. or link vid would be handy
When you install windows 7 it will obviously ask what partition youd like to install on, so choose new one.
Failing that,as t31 said you should be able to change boot order from the bios.