Hi,
First of all, please excuse me for by bad english as I am a french-speaking person 
Here’s my problem.
When I open the “Courrier new” font in Font Creator and then save it back without even changing anything, the bold does not look the same as the “original” Courrier new font.
The new font is much less … bold. 
I do not see any difference for italic and underline.
Has someone encountered this issue ? 
Any help would be appreciated !!!
When I open the Courier Bold font (which is installed) I cannot save it because it is in use. What exactly are you doing?
General advice is not to edit installed fonts. If you want to play with them, save the TTF file under a different name in another folder, then run the Autonaming wizard to give it a different name, then install it.
System fonts are especially tricky. If I open the windows font folder and delete Courier New Bold, Windows automatically reinstalls it, (or perhaps doesn’t delete it).
I backup all of my Windows core fonts in C:\My Fonts\Microsoft\