Clicking and Dragging enters a random formula - anyone know what is happening?

[Madraykin]

Hi All,

When I try and click and drag to select cells to copy (for example) if I have to drag to make the page scroll down Excel keeps entering this weird formula and it is really difficult to get rid of - does anyone know why it is happening (or what stupid thing I am doing???)

Pic of example can be seen here https://www.dropbox.com/s/f2u16zxwi5pzqeu/image001.png?dl=0

Any help appreciated - it's driving me nuts!

Thanks all!