"I sometimes, forget to update anti-virus software..."
All mainstream antivirus products can automatically update on connection to the web, whether over broadband or on dial up internet access, so you don't need to forget to update since the program can be set to do it for you.
They can also all be set to schedule a scan of your system, and most in fact have the auto-update and scheduled scan set by default. What stops this running is your PC not being turned on when the scheduled scan is set to run and it not being connected to the web when it is trying to download updates.
The fact that you said "The auto-update function is very useful for me" indicates that your new-found program is set to auto-update by default. So should your antivirus software, and if it isn't, if you give details of the product you use one of us will help you set it up properly.
It is worth pointing out here that all 'real time' moitoring of your PC has a performance hit involved where system resources are devoted to the real time moitoring.
It is also worth pointing out that antivirus, anti-spyware and anti-trojan software are all reactive and several steps behind the latest threats.
Basically what happens is a new virus/worm/trojan goes live and the antivirus vendors respond to it, often after several days or even a week or two. The better programs treat suspicious activity as virus activity, but no product is entirely effective.
Anyway, good luck with your program. I still think you should spend some time setting up your current antivirus software though, just to get the best out of it.