It would help us to help you if you can tell us what make and model the phone is, also which version of Android and which carrier you use.
First thing to try is a factory data reset, if that doesn't fix it (It won't if the trojan has rooted your phone, in which case the best option os to go to the phone maker's website and download the latest android specific to your phone, and reflash the phone. you should be able to do this using adb and fastboot from tour PC (Windows.Mac/Linux- there's versions of these tools for all PC OSes)