If you leave your computer on all the time and you are connected to a cable modem, make sure you have a very good router with a very good hardware (not softwaare) firewall. You may also want to consider using a McAfee software firewall as a backup.
I was amazed at how many times someonw tried to access my computer before I protected it. I was averaging hundreds of hits per HOUR!
When your PC is on all the time and connected to the internet, people can access it. I found files installed on my machine I never put there. It was very scary.
So, always practice safe computing by using the proper protection.
Mike