It is a small Internet.
The first thing you should consider is that Internet is not too big as you think. Most of us think that we are just a little point in a huge Internet ocean, so nobody would notice us. That is incorrect. Hackers use automated systems to search unsecured computers. They can detect you in a few minutes. Really.
Everything is worthy!
Secondly, you may think there is not any valuable thing in your computer but you are wrong. When you use Internet, you can enter your passwords to login the websites. Sometimes you can enter bank account with your login credentials. You can buy some products online with your credit card info. This information is not only stored on your computer, but also passes a lot of computers during transaction. Hackers can reach your passwords easily by using network tracking programs. In addition to this, the documents you saved in your computer include metadata which consists of your personal information. Hackers can steal your identity from that metadata. All the documents you have are valuable.
By the way, let me tell you something about your passwords. Do not set same password for all websites and tools. Always use different passwords. Use upper case, lower case, numbers and different characters (e.g. #$%&?*+) to make your password stronger. I personally set my passwords with more than 10 characters in order to make difficult to determine with password crackers.
Do not trust antivirus programs.
The third thing is that you should not believe your antivirus software in any case. It can not protect you always. You have to update your antivirus software and your operating system regularly. You better put a firewall to your computer. Security softwares are a must especially in Windows. However these precautions are for known threats, not for the new viruses. Keep them up-to-date!
Did you delete them for real?
The last but not least thing is that your files are never deleted from your computer when you delete them. I know this is complex but it is true. The files are stored in your harddrive separately. When you delete a file, it only deletes it's header, not it's body. Even if you click "shift delete" or even if you click "empty recycle bin". All files still can be reachable. Moreover, when you delete something from social media, they are not actually deleted. All social media websites store everything on big databases. They also keep a copy of all things.
Happy National Cyber Security Awareness Month :)