dxdiag.exe is a process belonging to Feardoor Trojan.
This process is a security risk and should be removed from your system.
If found on your system make sure that you have downloaded the latest update for your antivirus application.
Non-system processes like dxdiag.exe originate from software you installed on your system. Since most applications store data in your system's registry, it is likely that over time your registry suffers fragmentation and accumulates invalid entries which can affect your PC's performance. It is recommended that you check your registry to identify slowdown issues