A trojan, or trojan horse, is a seemingly legitimate program which secretly performs other, usually malicious, functions. It is usually user-initiated and does not replicate.
Additional Details
Trojan:W32/Inject is a large family of malware that secretly makes changes to the Windows Registry. Variants in the family may also make changes to other running processes. For a representative example of an Inject variant's actions, please see Trojan-Dropper:W32/Agent.FBB.
Inject variants may be delivered as part of the payload of other malware.