If you want your Windows PC to be secure, here are the essential steps.
1. Use a modern operating system. Sorry, folks—Windows XP simply isn’t secure enough for ordinary people to use today.
2. Keep your OS up to date and backed up. Turn on Windows Update and make sure it’s running properly.
3. Keep applications updated also. Remove unwanted programs that could represent a security threat.
4. Be suspicious of any new software. If you’re not sure a program is safe, don’t install it.
5. Set up standard non-administrator accounts for unsophisticated users. With a standard account a user needs to talk to you and convince you to enter the administrator’s password before installing any new software.
6. Use a modern browser. If you’re still using Windows XP and Internet Explorer 6, stop it.
7. Install an antivirus program and keep it up to date. I recommend the free Microsoft Security Essentials, which is available for download or as an optional update on systems where Windows does not detect an antivirus program.
And one final word: Don’t be paranoid.
The article (Do you really need antivirus software?) is more substantial than what I’ve quoted above. That said, it’s not long and well worth your reading time.