I understand that the bootstrap program runs initial diagnostics and then calls the bootloader from a fixed location.
When my had just two windows OS(example WINDOWS 7 and WINDOWS XP), a windows bootloader used to provide option of switching between the two operating systems. Once I installed Ubuntu (and grub becomes the bootloader), I am first asked to choose between windows and linux and then once I choose windows, I get the windows bootloader screen to choose between two windows.
My understanding was that there can be only one bootloader at a fixed location which is known to the bootstrapper. So in this particular case how do the windows and linux bootloader coexist?
Bootloaders can exist in multiple places.
There is one MBR (Master Boot Record) but there are multiple partitions. Each partition can have a bootloader in its Volume Boot Record (VBR) - the first sector of a bootable partition. You can have a bootloader on the MBR and/or within a VBR, so the MBR will chain to the secondary bootloader in the VBR. This is how you see Grub first, then Windows 7. Grub is on the MBR. But if you reinstall Windows, it will stomp on the MBR and put its bootloader back.