To stay safe, go to sites that offer movies that you know are legitimate. These would be things like Hulu, network sites, Youtube, and others. These are legal and they are safe without huge malware issues.
If you hear about a site that offers 123movies on it, search the internet for real reviews about the site. Find out if it's a legitimate service and if people have any comments about virus problems.
If it's a movie that you know is still in the theater, don't watch it. You know it's not legal and you don't know what the site is doing to your computer.