Select View > Layout
Make sure Show preview pane is not checked.
Click OK.
The preview pane should never be on, it is a huge security risk as it automatically opens mails as soon as you click on them, and as you have found out you then can not delete suspicious mails without first opening them.
thanks a million for both answers. have gone with the preview pane option -have never heard of it and am so grateful for the explanation. many, many thanks to boyh
And if I may add, if you right click the toolbar on top and choose 'customise' you can add an 'preview icon' to the toolbar so you can instantly turn the preview on/off as your heart desires. That saves you having to go to through the View tab all the time.