If you have a dropdown list connected to the Company field with Type Ahead enabled and Automatically Add to Dropdown list enabled, that will happen. You need to edit the type first in the dropdown list and then you can correct the actual field record. You did not specify which version of ACT! you are using, the default since ACT! 7 is not to have a dropdown list for the company field, however either ACT! 5 or 6 or perhaps both did have a dropdown for the company field and it had automatically and and type ahead enabled.
Roy Laudenslager ACT! Certified Consultant Techbenders email@example.com