I have a SATA hard drive with an MS DOS MBR partition table, an ext4 Linux-based OS, an NTFS Windows 7, and an extended partition. I was happily using my ext4 partition for quite some time, but grew anxious to return to my extensive Steam games collection. So, I then made the mistake of installing Microsoft Windows 7. Now, the system loads the MS bootloader instead of GRUB, and when I try to change the default OS, it only registers Windows as being an OS on the computer. I would hate to think that in all of their "expertise" the MS crew couldn't even write something to recognize that ext4 partitions exist. So, how can I change the main bootloader back to GRUB? I could always load both OS's with GRUB before, but now I don't know what is going on.
Use the tool Boot-Repair.
Boot-Repair lets you fix these issues with a simple click, which (generally reinstalls GRUB and) restores access to the operating systems you had installed before the issue.
The easiest way to use Boot-Repair is to burn one of the following disks and boot on it.
Video demonstrating the use:
Easiest option, in my opinion, would be to boot a live Linux disc (Ubuntu may be the easiest) and run Boot-repair.
As a summary: