Hi,
Welcome to Microsoft answers.
Since when are you facing this issue?
Were you able to open .csv file before?
Do you able to save other files within the computer?
Did you make any recent changes?
I suggest you to contact Microsoft Office, and would be better suited in the Microsoft Office Discussion Groups.
Please access the below link and find a community that will offer you a better support.
http://www.microsoft.com/office/community/en-us/FlyoutOverview.mspx
Let us know if you have any issues.
Thanks and Regards,
Azam – Microsoft Support.