How do I correct unwanted customization of my unrooted samsung galaxy s5 sm-900v?

[Android Central Question]

For in rooted Samsung gal s5 phone sm-900v: I tried wondershare dr. fone trial to see if I could retrieve data, after factory reset done because of the Verizon bootloop. It did something that caused it to have a custom unlock which cannot be erased by another factory reset. It was unlocked before, but not custom. Now it gives security warning to restart because of unauthorized changes--every time I power on. How do I resecure my phone -- from this apparent malware?
To bootload flash of firmware using Odin, I would have to use windows vista home premium; if this is my only option, then how do I make sure the firmware md5 is valid and correct. (I want to make sure rather than be sorry, again.)
[I may consider a completely new custom rom that is SECURE through rooting, if necessary.]

 

[Rukbat]

Download the firmware from a trusted source (Sammobile doesn't happen to have firmware for the V). This XDA thread seems to have the official ROM, although the 900V subforum has a lot of ROMS.