If you hear about a site that offers movies 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 123movie 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.
You can also watch movies online by renting them. There are services like Amazon and iTunes that offer rentals that you can stream to your computer. These won't have legality or malware issues with them as they are real services.