Here are a few security tips that over the years, that over the years I have found.
1) Do not use the same password for every cashier. Making them have separate passwords can enable you to track who was manning the register. Furthermore, have them regularly change it, including your own. A good tip here is write in your diary say once every year that all passwords are to be changed on this date.
And please do not use your name, everyone who works in the shop knows that.
If you want to check out how good, your password is, check it out here.
Do not put your password exactly in but a slight variation say instead of "aa" put in a "bb"
2) Check any transactions that were voided or canceled. This is a common method used to make stock disappear from a store. An alarm bell should ring, if someone returns an item that you have no record of selling.
3) Set up password-protected security on all computers. The computer without security is the one people wanting to hack are looking for.
4) Regularly go over your stock reports to make sales figures match with the computers.
5) Set up security cameras not only covering the shop but also the cash register too.
6) It is not enough to check your computer for virus; you need to check your router/modem. This is what actually takes you to the Internet. What happens is some virus will change the setting in your router allowing them to take control of all of your Internet usage.
I would suggest you test your router too.
I hope this helps.