There are some ways to Inject another process:
1.Change Register.When a Process started, Windows will load all DLL file Recorded in HKLM\Software\Microsoft\WindowsNT\CurrentVersion\Windows\AppInit_Dlls.
2.Create Remote Thread.
3.Use reliable process inject.
4.APC inject.
5.Message Hook.
6.Import Table Inject.
7.LSP Inject.