Question about reinstalling GRUB

[Bill Stanley]

I have an older (but still good) computer that dual boots Ubuntu and 
Windows.  The Windows partition needs the Windows OS to be reinstalled.

Here is the question.  When I reinstall Windows, I know that the GRUB 
boot-loader will be overwritten by the new install of Windows.  At that 
time, the computer will only be able to boot Windows.  What is the best 
way to reinstall the GRUB boot-loader.  Saving the existing GRUB 
boot-loader is probably inadvisable.   As such, generating a new GRUB 
boot-loader probably will be needed.  What is the best was to do this?

Bill Stanley