Have the file extensions changed to .exe or .zip? Sounds like you're infected. Try AVG Free AV...that should find the culprit.
AVG Free Advisor: AVG Anti-Virus Free

Download AVG, update it, scan your computer.
Search the forum & you'll see other people have had this problem...AVG has fixed things for the majority

If you need more help, try one of the sites below.

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SpywareInfo Forum FAQ - SWI Forums

Sad news is that you've probably lost your files, unless you'd backed things up before this happened.

Always make sure that you're using a good AV program. Keep it updated & scan all files that you've downloaded before you open them. There are many infected files on the network.