A computer left unattended is less likely to be infected with computer spyware and viruses then one which is used often. This is because an inactive computer does not seek out new information on the Internet or input new data into it's systems. At most an unattended computer will do is to download anti-spyware and antivirus updates which do not pose a problem.
What poses a threat to computer security is actually human activity. It is the user who through his or her actions on the computer either knowingly or unknowingly invites a spyware or virus infection.
So the user - that's you - actually poses a greater threat to your computer security that the computer itself.
An antivirus or anti-spyware that is installed - and forgotten about - will only pose a greater threat to the computer in the long run. This is because anti-spyware and antivirus are sophisticated pieces of software that need proper looking after. Neglecting to maintain the anti-malware in proper working order will result in lesser protection from them in the long run. The result is the computer will eventually be infected with viruses and the user will be none the wiser.
It is therefore absolutely crucial for you to learn how to maintain your security software so that it can do it's job properly for you.
First off, there is no such thing as a "best antivirus in the world". No reputable antivirus maker ever makes such a claim. That is because even the so-called "best" antivirus can be defeated by viruses. So spending a little fortune on expensive antivirus and anti-spyware products is no guarantee you will have peace of mind. Ignorance is no excuse. In fact the more ignorant you are, the more likely your computer will be infected with viruses and spyware. That is because an ignorant user is more likely not to notice a malfunctioning antivirus. Many users have expired or disabled antivirus and anti-spyware sitting idle in their system tray offering no protection whatsoever. Yet these users linger under the impression that the software is protecting their computer. The result is the computer is left open to infection.
The only solution to such a problem is education. A little education goes a long way - and here at Computer Cadet such an education is free. What more can you ask?