Excel 2010 with same format in all cell

Hi!
Excel 2010 suddenly got the same format in all cells.
If I open an old or new sheets so are one and the same format to all cells and I mean really in all cells.
For example, I enter 457889 into a cell, the answer 4578.89, I write 456 the answer is 4.56 then the cell content controlled to be mandatory two decimal places at the end.
If I open an earlier document, before this happened, so everything looks right out and all numbers etc are correct, but if I try to add new values to an old document that will be the format as above with the obligatory 2 decimal places and as you can understand the result disaster.
To change the cell formats can not get the same results no matter how I do.
I can not find where this oddity comes from or how I should get rid of this, as you understand it's completely stopped my work.