How can the risk of unauthorized computer system access be reduced?

Most users are interested in taking steps to prevent others from accessing their computer. Whether it be to protect yourself from malware or to ensure that your private information is safe, having a secure computer can definitely provide peace of mind. The following sections detail many ways you can secure you computer against others. To proceed, you may read through each section or choose one that interests you from the below list.

1.  Passwords

2.  Get a hardware or software firewall

3.  Operating system and software patches and updates

4.  Malware protection

5.  Run system scans to check for vulnerabilities

6.  Know how to handle e-mail Alternative browser

you have very good answer to your own question.

I totally agree with your answer, Ms. Ghada Eweda

I may add two points which are :

not to open any unknown files you may receive from strangle people online..

don't plugin unsafe pen drive into your USB port of your computer.

my two cents: avoid sharing folders, especially if you are in a work environment (domain); if you need authorized people to have access use ftp.