In the last few weeks, I've had 2 clients who got infected with the Cryptolocker Ransomware. A ransomware is technically not a computer virus because it doesn't spread itself to other computers, but it's effects can be worse. Ransomware in general asks you to pay money in return for getting your computer's functionality back. The Cryptolocker Ransomware is the worst malicious program I've encountered because it encrypts your files including any backup files you have on an external drive (if you had the external drive connected when the computer got infected). And there is no way around getting your files back unless you pay their $300 ransom (you have either 72 hours / 3 days or 96 hours / 4 days to pay the ransom or it will delete your encryption key and you will not have any way to decrypt your files). The ransom must be paid by MoneyPak or Bitcoins. Once your payment is sent and verified, it will decrypt the files that have been encrypted.
To protect yourself from this ransomware (and others like it that is sure to come in the future), use an online (cloud) backup service. Up until this time, I never strongly advocated for an online backup service because an external drive backup is suitable for many, especially for those who are still wary of storing their data on the cloud (which I can assure you is really safe these days and if you have an email account, you're already storing your data on the cloud anyway).
Other than protecting your files from this Ransomware, the main reason I recommend backing up on the cloud is convenience. You install the software once and forget about it. The service I recommend most is Carbonite because of their flat rate pricing ($59.99/year). Although one caveat is Carbonite will not backup your video files unless you explicitly tell it
to do so --- and you have to do this every time you upload a video file. If you want automatic backups of video files, you can subscribe to Mozy which automatically backups your video files, but they don't offer flat rate pricing, so it could cost considerably more if you have a lot of pictures, music and video files ($5.99/month for 50 GB or $9.99/month for 125 GB). Another option to consider is a cloud file sharing service like dropbox ($99/year for 100 GB) and put all the files you want to backup in your dropbox folder.
Another great benefit of an online backup service is they keep previous copies of your files (usually 30 days worth), so if you accidentally delete a file, or make the wrong changes, you can restore to an older copy.
For more information about the Cryptolocker Ransomware, see http://www.bleepingcomputer.com/virus-removal/cryptolocker-ransomware-information.