excel and word files will not open

Today a number of staff reported to me that in the afternoon they could not open some excel and word files because the "file format or file extension is not valid...verify no corruption"  This does not apply to all files.  Some excel files modified and saved this morning still open in the same network folder where others do not.  I hope this is not a virus. Help
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Mike-LFCAuthor Commented:
Malwarebytes does not find any problems but spent a lot of time in the windows installer folder.  There is one workstation where windows installer opens and wants to run an msi file.  Wouldnt a virus like encryptolock effect all files in a network folder.  Files that wont open are the same on all workstations.
Mike-LFCAuthor Commented:
Malwarebytes spends a lot of time in the windows installer folders on all of the workstations.
Davis McCarnOwnerCommented:
What version of Office (2003, 2007, 2010, etc.)?
What happens if you copy one of those files to a local drive?
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☠ MASQ ☠Commented:
OK let's be pessimistic.

This could be encrypting malware - if so the source machine will still be able to open the files that everyone else sees as corrupted because the affected machine is running on-the-fly decryption to hide its presence from the user. That ability will help you confirm it (when you find it)

Encryption malware can damage any data files that the infected machine has read/write access to.

Are you seeing changes to file extensions with a common but random string being appended to the file name?

How many users?  Any unusual traffic?  Bear in mind one machine will be sending all the read write instructions.

Most importantly do you have back up?  Crypto's can usually disable ShadowCopy so that alone isn't helpful.
Mike-LFCAuthor Commented:
I have found that is it Cryptowall 3.0 with a ransom note for payment in every folder.  Apparently it actually started last week, disabled shadowcopy and messed up our Thursday and Friday tape backup jobs.  Is there any way to unencrypt some very important excel and word files.
☠ MASQ ☠Commented:
Sorry, no. - Short of paying the ransom and hoping they'll send you the private key for the encryptor there isn't a way currently to do this - it uses a non-standard AES256 encryption so brute force is realistically impossible.

Did you track the source down?

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Microsoft Excel

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