Trojan.Agent refers to a generic Trojan malware detection. It is used for detections that are either associated with no specific malware families or not enough information is available to pinpoint the malware family.
Trojan.StartPage is the generic detection for Trojans that continue to change the start page of a browser.
Trojan.StartPage changes the startpage of the affected browser and will keep doing so. The startpage will be set to a site of the threat-actors choosing. So the browser will open with a site different from the users preference.
Malwarebytes protects users from Trojan.StartPage by using real-time protection.
Malwarebytes can detect and remove Trojan.StartPage without further user interaction.
After removal, you canmanually change the startpage back to how it was before the infection.