SE G900 troubles!

Hello. I have a G900 with a big trouble- i think a virus which drains very fast any battery i put in my phone(i tried at least 5 batteries untill now) and if i don`t use the phone it discharges in half a day. A week ago i didn`t have this trouble, the phone stayed charged for 3 days in minimum usage. I tried to master reset it but I don`t know the security code(i`m not the first owner). Do you have any idea how to hard reset the phone or if i can flash it from home? I think to update the firmware with Seus but i`m afraid it will lock on the vodafone from where it has been bought(now it`s unlocked and i don`t know how it has been unlocked.
Any opinion is welcomed! Thank you all!

Disable 3G in settings.