Here you can find all the necessary files for playing on xMU..

Additional software components may be required to run the game on your computer , for example:
.NET Framework и vCredistx86

If your antivirus "swears" at the MuOnline game files, considers some of them a virus or suspicious, and deletes them, then just add the folder with the game to the antivirus exceptions. Game files are not dangerous and do not pose a threat to your computer. However, some antivirus programs and firewalls may mistake the internal structure of MU Online files (an anti-cheat system) for malware.