To schedule a virus scan of your system you have to first check the Scan My Computer at a scheduled time box. The section in the middle displays the current schedule and when the next system scan will be performed.
You can edit the scanning schedule by clicking on the Edit button. You can choose to schedule a scan Daily, Weekly, Monthly, Once, At System Startup, At Logon or When Idle.
Depending on the selection you choose, your options for the rest of the schedule will change. Daily will ask you how many days to wait between scans. Weekly allows you to choose which days of the week scans should be done. Monthly lets you choose what day of the month to initiate a scan and so on.
The Advanced options let you select an end date for the schedule and the Show multiple schedules checkbox lets you choose to create more than one periodic schedule.
I recommend setting up at least a weekly scan. If you leave your computer on overnight it is best to choose a time in the middle of the night when the scan won't affect your ability to use the computer.