Sometimes captcha fails to load on admin screen or if you want to disable it manually then try following steps to do so. Magento Disable Captcha Admin Login
Step 1: Login to your hosting control panel and go to following path app/etc/modules/ open & edit Mage_Captcha.xml
Step 2: Should have true value within <active> tag, just replace it with false as shown. Change the value from true to false and save file.
Result: Captcha on Magento Admin Screen is disabled now.
Please Note: If you still have captcha on admin login after completing these steps then try clearing magento cache. If you would like to clear cache go to var/cache and delete all files.