Hi magpie and welcome to the Forum .
To select a "Thorough Scan" you can simply click on the 'Image' of a Hardrive in the Silver panel skin , and a pop up will appear with options for quick , standard , or "Thorough scan " . Select with a sliding control .
Also , another way ; find the drop down Menu , in the case of the above mentioned skin , located in the Top-Left hand corner.Click on it . Mouse over "Start Scan" >Select Scan Area . Then , "Select Scan Level " , either quick , standard , or "Thorough scan " . You also have the option to scan Archive files mixed in to selections.