Excel adding decimal places

The oddest thing, a bug with Excel that I've never seen before.

In an existing workbook, when I type '90' into a cell and press return it shows up as '0.0009'.

Its nowt to do with the cell formating, when I copy the formating from a kosher cell it does the same. Tried setting the cell formating to number General and Number (with no decimal places) but it makes no difference. I've never had this problem before with this workbook.

So, I'm having to resort to copying and pasting the value I need from elsewhere.