There is currently an ongoing situation where users may encounter an error when trying to open Office documents. The error can happen opening any Office file type, not just Excel files. For Excel file types, the error says: "Excel cannot open the file [filename] because the file format or file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file."
For Microsoft Word, the error may read differently: "The file cannot be opened because there are problems with the contents" or "The file [FileName] cannot be opened because there is a problem with the contents".
We have confirmed that this can also affect PowerPoint files, AutoCAD files and JPEG images.
This problem has been confirmed to be caused by malware on the affected machine. There are now two known variants of malware which causes this problem: Win32/Crilock.A and Win32/Buma!rts. They have both been identified as a new family of ransomware.
In order to clean your machine, run Microsoft Safety Scanner (http://www.microsoft.com/security/scanner/en-us/default.aspx). If infected, Safety Scanner should clean the virus from the system, however it will not repair corrupted files. You will still need to restore those from a backup. A detailed analysis of affected files submitted to Microsoft for investigation has revealed the files are encrypted with a private and public key. The files cannot be recovered without the private key, which is more than likely held by the attacker. The premise of ransomware is such that if a person pays the ransom the key is provided to "unlock" the files.
Microsoft PowerPoint Support Blog: http://blogs.technet.com/b/bgp/archive/2013/09/09/3595491.aspx