Are the two using the same Windows bootloader? If so, you may
want to look into BCDEdit.
If not, you could try setting the startup disk directly from the boot
option menu. Hold down the OPTION key when booting. When you
are presented with a choice, hold down the CONTROL key first and
then, whilst holding it down, select the Windows 7 option and hit
RETURN. Then you can release the CONTROL key. That should, if
all goes well, boot into Windows 7 and make Windows 7 the default
boot OS.