Customer data
If you store customer data on your PC, you should put an extra effort in security. Even if it is "only" email-addresses you store, keep the data locked as much as you would like your data locked on somebody else's PC.
If you don't have a firewall, get one. You can download a cheap or even for free version just about anywhere on the net (see the links section). Also make sure you are not vulnerable to viruses which may open a "backdoor" to your PC and your data. An up-to-date virus scanner is something you should have installed with or without running a business anyway.
A good way to avoid having to have extra security is, not to store sensitive customer data (e.g. credit card numbers) at all. If you use a PSP, you won't even see this data and you have one less thing to worry about.
Whatever you store, ANY data is worth protecting, and it should go without saying that you don't share the data with business partners or ANY third party without the explicit consent of your customers.