Is BitDefender a reliable antivirus software?

My boyfriend’s Dad uses a purchased full licensed version of BitDefender and I have always simply used the free version of Comodo Firewall, yet I have little to no problems with viruses and he is overrun by them! I am constantly having to clean Trojans from his computer and what I don’t understand is why such a highly acclaimed anti-virus software like BitDefender, that he pays for, misses 99.9% of viruses?!?