How do I reset my iMac like straight from the factory

To factory reset an iMac without a login or keyboard, you can try the following steps:

1. Boot the iMac in Target Disk Mode: Connect the iMac to another Mac using a Thunderbolt cable. Power on the iMac while holding down the T key. The iMac should appear as an external disk on the other Mac.
2. Reinstall macOS: Open the Disk Utility app on the other Mac and select the iMac disk. Choose "Erase" and follow the on-screen instructions to erase the iMac's drive. After erasing, quit Disk Utility and follow the on-screen instructions to reinstall macOS.
3. Restore from Time Machine Backup: If you have a Time Machine backup of the iMac, you can restore the backup to the iMac after reinstalling macOS.

If these steps don't work, you may need to seek assistance from an Apple Store or a professional repair service.

Back up all of you data to iCloud. Then reset Mac to factory. Then download Yosemite.

Boot the computer normally. When you get a screen, go to System Preferences under the Apple Icon, left hand side of the menu bar. Go to start-up items and uncheck or remove the offending game.
The next restart should be normal. You could always delete the idiotic thing if it persists in taking over your Imac. Go to Applications, click on it and drag it to the trash. It might help to shout Die! Die! Die! as you do this.
Won't do a thing for the Imac but you might feel better.

try this Recovery
Solution
Factory data
reset
How do I perform a
Factory data reset?
Resetting the phone does not erase
any system software updates you have
downloaded or any files on your microSD
card or internal memory, such as music
or photos. And so, if you want to erase
all the data on the internal memory while
resetting the phone, select the check box
of Erase internal memory.
•
Recovery
Solution
Hard Reset
(Factory Reset)
How can I perform a
factory reset if I can't
access the phone's
setting menu?
How to perform a Hard Reset (Factory Reset):
1. Turn the power off.
2. When the phone is switched off, press and
hold the Volume down key + Home key
with your left hand. Then press the Power
key while pressing Volume down key +
Home key.
3. LG logo would be displayed in 1~2
seconds then the phone would re-boot
showing android robot animation. Finally
the factory reset would be processed.
Attention:
If a factory reset is performed, all
installed applications and user data will
be erased. Please remember to back up
any important data before performing a
factory reset.

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0CDIQtwIwAQ&url=http%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DROh0bPHYxqk&ei=LSk9U9_zKfepsASNqIGIBQ&usg=AFQjCNFTgvSrJrnGq4_Iwv33dsTUJiWgUA&sig2=3Rr1q2QOXA2oShUAHvplmQ&bvm=bv.63934634,d.dmQ

• 
  Press the Menu Key > Settings > Privacy > Factory data reset > Reset phone > Erase everything. But all the personal information will be deleted when u reset the phone.

Easy reset the router to factory setting....if you do not have or know the password then that's another problem ....

if you reset the router to factory settings, leave the username field blank, and use "admin" for the password

If you are running Leopard, reset the printing system by control-clicking in the list of printers in the Print & Fax preference window. Then reinstall the drivers.
Apple Menu > System Preferences > Print & Fax. Control Click in the left area which shows the list of printers, and choose "reset the printing system"
macs2u

http://superdrive.cynikal.net/
If not download VLC player. (Video Lan Client)