I already had installed Windows 7 on
D: partition. I installed Windows 8 Developer Preview on C: drive but I can't dual boot Windows 7 and Windows 8, it directly boots to Windows 8. My D: partition contains all Windows 7 files. How do I access Windows 7 again?
Download Visual BCD Editor. It comes with Dual-boot Repair tool.
First run Dual-boot Repair -> "Automatic Repair" - this will fix boot environment (if any errors).
Then run Visual BCD Editor. RightClick -> Select "Create missing Windows loaders" (this will add boot entry for Windows 7 and any other installed Windows system XP, Vista, Windows 8).
To set Windows 7 as default: click on Windows 7 in treeView - in Object tab - Set as default loader.
Reboot you should have a dual boot menu.