Since this seems really hard to document and for some reason all tutorials suggests that you need to install another Linux distro...

  • Install OpenELEC as usual
  • Boot GParted Live without starting OpenELEC
  • Resize the last parition to 7-10Gbytes
    If GParted asks select no if it wants to fix your partition table
  • Install Windows on the remaining part of your HDD
  • Install EasyBCD
  • Select "Add New Entry" in EasyBCD
  • Choose Linux/BSD as Operating System
  • Select the smallest partition as that's where OpenELEC's kernel is
  • Make sure "Use EasyBCD's copy of GRUB" is not selected
  • Add Entry
  • Save by pressing "Edit Bootmenu"
  • DONE!

Thanks goes out to heimdal @ OpenELEC forums...
http://openelec.tv/forum/66-multiboot/63938-dualboot-win7-openelec-3-0-0-my-solution-guide