If your antivirus 'swears' at the MuOnline client files and considers some of them as a virus or suspicious and deletes them, then add reported client files to the antivirus exceptions. Game files are not dangerous and do not pose a threat to your computer.
Please check your system requirements to ensure your system is suitable for your selected game.
Pentium 4 – 2.0 Ghz or higher
1 GB or higher
3D graphics processor
DirectX 9.0c or higher
Hard Disk Space
2GB or higher
Make sure you have the latest drivers installed. The links below will help you update the drivers best suited for your system.