That's the F-Secure automatic system scan, I think. If it bothers you, either reschedule it for some other time, or turn off scheduled scans and do it manually whenever you are going to be away from the computer for a while. You can change the settings for automatic scans under Tools - Scan Settings - Scheduled Scans (at least I assume that's what the options in question are called in English, my version is in Swedish). Unfortunately, I don't think there is any way to turn off the scan mid-progress, except by killing the process from the Task Manager. That's my biggest complaint about F-Secure: there is no interface for the user to control, monitor, or even be aware of, scheduled scans running.
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