Just bought the game tryed downlading says it has a virus and when I try to log in I get a black screen

Just bought the game tryed downlading. says it has a virus
( Accessed file is infected
File name c:\Users\Emma\AppData\Roaming\.minecraft\bin\natives\lwjgl.dll
Threat name: Trojan horse Crypt_c.FMC Detected on open )
and when I try to log in I get a black screen. I am not that good with computers and do not know if I am reporting my issues right.