Unplug pc from the wall.
Remove the battery on the motherboard.
Leave for a few seconds
Replace battery.
It is unlikely that a CMOS reset would alter the boot config though unless it had already been altered by the system builder.
Try the CMOS reset by all means and when you have and if you still have no luck try this:
Whichever drive is your boot drive (SSD I presume), remove all other drives SATA cables and just leave the boot drive plugged in. IF windows still will not boot it could well be a corrupt install.
Either way, you really need to get into your BIOS. Stupid question but you sure you're pressing the .del key on your number keypad and not the delete key above 'backspace'?