Cell displaying wrong figure

Hello all,

I have a problem which seems to be affecting all my Excel worksheets whenever I enter an amount into a cell.

If I enter a number ie 100 then tab to the next cell, it returns a value of 1. If I enter 100.00 I then get the desired figure in the cell of 100.

Initially I thought the cell format had changed but I've tried General, Number and a few others but nothing changes.

I then noticed that this is affecting all my spreadsheets

Any ideas???