If you are looking for updates, Support has ended for Office 2007.
To use the newest features and get the most up-to-date security updates, we recommend you get the latest version of Office. If you are looking for updates, Support has ended for Office 2003.
Copy the contents of the spreadsheet to a new one, and deleted the old one (it wasn't a particularly important spreadsheet, thankfully), and everything was fine.
I can only assume that somehow the file had become corrupted.
That probably turned the cell into a text format cell, even though technically the cell was still a number and listed as one under the formats.
Internet Explorer 11 Release Preview brings the "best in class" spelling engine and dictionaries used by Microsoft to the browser.
Internet Explorer 11 Release Preview also supports autocorrection or "correction-while-you-type".
I also have had this problem with a very large spreadsheet that just stopped updating itself over the weekend, but having checked the above solutions, setting were already set to automatic, and sheet is too big to rebuild, so I was at a loss.
My solution was to find an old version of the same excel spreadsheet which was not having thses issuesbefore I archived it, which is of similar size and complexity to the current version.