Security analysts are warning computer users that a virus that took down computers around the world last week, could strike.

Hackers discovered a weakness in Microsoft Windows Friday, allowing them to access millions of computers.

Microsoft is working to contain the damage, offering free patches for customers, but they say computers shut down over the weekend still face the threat.

The virus locks up computers and then demands a ransom be paid.

The attack is believed to be the largest in history, with 150 countries impacted.