I have a newly built system and i have a hard drive from an old computer. Windows wont boot from the old hard drive (due to drivers issues and possibly boot.ini file, i've tried) so my question is if there is a way to repair the windows from the old hard from the new drive to allow the old hard drive to work as it used to?
More aboutharddrives windows possible
You do not need to reformat, but you do need to reinstall Windows.
You've made a huge hardware change to the system that windows on your old hdd 'knows' so its sulking, save any files you want to keep on an external drive, then reinstall windows in the new build so it knows what the hardware is