Do you have another linux machine?
e.g. you could setup a vm (download vmware player for windows and a minimal debian iso image, install vmware, select new virtual machine, select debian image as virtual cdrom, startup machine, install debian (this is almost straight forward 
)
If you donāt want to install vmware (or another virtualizer), you could use a live DVD (whatever, ubuntu, debian, fedoraā¦ doesnāt really matter)
After you got a GNU/Linux bash, you have to mount the openhabian sdcard, chroot to openhabian and set new passwd.
Maybe this is a bit tricky for a non GNU/Linux userā¦ but itās not really hard work. You have to get information, which device is the correct one. For this, connect the sdcard after login to the new bash. after a few seconds, type (without ### part)
dmesg ###get last messages from kernel
and find the last devices /dev/sdā¦ (something like sdc2, sdc3, sdd2 or so, depends on your builtin hardware, how many harddisks and so on)
Then mount this device:
su ###become super user
mount /dev/sdc2 /mnt ###mount device to folder /mnt
take a look at the filesystem:
ls /mnt/ ###list contents of /mnt
there should be at least etc/ bin/ sbin/ usr/ home/ root/ and some more. If not, this is the wrong device, so unmount it by typing unmount /mnt and try another /dev/sdcā¦
if you found the right one, use
chroot /mnt/ ###act as if /mnt was /
to āloginā as root into openhabian.
then use
passwd openhabian
to set a new passwd.