I have a dual-boot Windows/Ubuntu, my default OS is Windows :(. So, when Windows is started and I need to go to Ubuntu, i need to choose him on Grub, of course. I would like to make a Windows shortcut to reboot on Grub and start on Ubuntu. There is a way to do this?
The simple answer is no.
Imagine it this way: when you boot your computer (the machine that you can hit with a hammer when you are angry) it's going to ask your drive for instructions. If only windows is present: boom, you boot-up on windows. However, when grub is present, grub will work as kinda of a tiny system that will let you choose further. Whilst you are in Grub, Windows hasn't done anything yet. In fact Windows doesn't even know Grub is there.
What you can do, however, is configure Grub to boot straight to Ubuntu. Alternatively you can use a Virtual Machine.
The solution by Hate Linux (plus iReboot) works if your Windows bootloader starts first. If you want Grub to control your boot process, there is another tool: grub-choose-default. However, to use it from Windows, you have to make your
/boot partition accessible from Windows. This usually means formatting it as
ext2 and using EXT2IFS. Also, I haven't found a PPA for this tool with packages for recent Ubuntu versions.
See also the following questions on Unix.SX: