Windows users may need to go to a site set up by the FBI and make sure their computer is not infected by a particular virus.
The agency says a group of hackers installed software on more than 570,000 computers worldwide.
The FBI has been keeping all the infected computers online since they caught the group in November.
But now they want the estimated 350,000 still infected users to go to the web site they set up to get the problem fixed.
Windows users are advised to visit www.dcwg.org, which will show you whether you've been infected and guide you through a quick fix.