Before you mail holiday gifts to far-away friends and relatives, back up your most important computer files. There's a scary new computer virus called CryptoLocker that was spreading like crazy in the U.K. last month and is now crossing over to infect U.S. computers.

The National Crime Agency in the U.K. issued an alert last month saying that hackers have targeted "tens of millions" of computers.

CryptoLocker is a form of a virus called "ransomware," meaning hackers do something bad to your computer and then demand money to reverse what they've done. In this case, CryptoLocker encrypts the files on your computer. Then you get a pop-up notice on your computer telling you that you must pay if you want your files back.

Sometimes the hackers want $100, sometimes $300, and sometimes they want up to 2 bitcoins. They ask for bitcoins because they are difficult to trace.  Sometimes the virus gives you the option of paying through MoneyPak, a site that offers pre-paid credit cards.

You might get 100 hours to pay. If you pay, maybe your files will be set free. Maybe they won't. Maybe you'll one day be asked to pay again.

Much of the time, the virus comes as an attachment to an email. The attachment is often a fake FedEx and UPS tracking notice, the Homeland Security Cybercrime unit warns. That's particularly dangerous during the holiday shopping season when many people are shipping packages.

The good news is that authorities have been finding and taking down the computer systems sending out CryptoLocker emails. Reports on Monday indicated that some 138 of these systems have been found and blocked.

But infections are still running rampant. Even a police department in Swansea, Mass., was infected and paid 2 bitcoins in early November, worth about $750 at the time, reports the Herald News' Brian Fraga.

Antivirus software will help. But the best defense is to avoid opening attachments and to back up your files, experts say.

Here's what the ransom message looks like:

CryptoLockerMalwarebytesCryptoLocker ransom message