All forumlas return 0 in spreadsheet

Hi,

I'm using VBA to do some reasearch on a few logs I've got. Frome time to time I experience a pretty big problem. The problem is:

When I start excel, excel refuse to calculate any formula at all. If I for example enter =today() in a cell, it just returns 1/1/1900 or something like that. Also if I for example enter =10/5 which really should return 2, I get 0.

This makes all my formulas return 0...

The only solution I've found to this is to close the spreadsheet, create a "New Microsoft Excel Worksheet.xls", open it. Test that =today() works, and then reopen my original spreadsheet.

The formulas then start working...

What may be the problem? I'm pretty tired of this now :)

Thanks in advance for any help!