You should enable Security so that the credit cards you enter are more secure.
Click Tools > Security to set this up.
At minimum, you should assign write permission to the Default Group and create one user in the Default group. This will force your to log in with a user name and password when starting the program. If more than one user is using the program, you can create various groups with different permission levels. Each user will only have permission based on the group he or she is in. Only the users who are in a group with permission to the credit card domain will be able to view and charge cards.
Click here for more information on Form Level Security.
Be sure that your computer is secure. You have updated virus protection, a firewall spyware protection and have the latest updates and service packs for Windows.