There is a nasty Windows virus doing the rounds. It is generally referred to as the CryptoLocker virus and it is a category of virus or malware known as Ransomware.
It comes in as part of an e-mail that appears to be from a valid source and it is generally disguised as a ZIP file or a PDF, but it is actually a cloaked executable. Once you click on it the executable begins to run and starts encrypting all your files, including files on mapped drives and any connected USB devices.
Then you are told that you can get your now useless files decrypted only if you send money.
If you don’t send money within the specified time, typically about 72 hours, then the decryption key is destroyed and your files can never be decrypted. So unless you have off-line backups of your files then you either pay the money or do without them.
There is more information on Wikipedia here.
You could also do a Google search for CryptoLocker to find out more.
Be careful what you click on in e-mails that your receive.