Just spotted this in the wrong area so moved it. Not knowing whether you have the 2009 or 2010 version I will try to explain both.
If your taskbar icon is a red square with a white M inside then it's the 2009 version and the following apply:
Double-click the taskbar icon to open SecurityCenter
Click Advanced Menu (bottom)
Click Configure (left)
Click Computer & Files (top left)
Click Advanced (right)
Click Scheduled Scan (left)
If your taskbar icon is a white-rimmed red shield with a white M in the middle then it's the 2010 version.
Double-click that icon to open SecurityCenter
Click Realtime Scanning to expand
Click Scheduled Scan Settings.
Hope that helps and sorry to take so long to answer.