Question : Error Loading Operating System

About a mouth ago a customer brought me his Laptop (Dell Inspiron 6000) and at that time he had two issues: one was that he had allowed the antivirus software license to expire and so the system was chocked full of viruses, malware, etc.  So, first of all I ran a test on the hard drive to make sure it was good and then formated it and re-installed the OS.  I also installed Avast antivirus Software and Malwarebytes anti-malware (both free version).

     Also his adapter had quit working and the one he had purchased wasn't the correct one.  It would run his computer, but since the amps didn't match it would not charge the battery on his laptop.  The laptop was calling for 19.5V & 4.62A and the charger was putting out 19.5V & 3.34A.  Now, a month later the laptop will not boot up. He gets an "Error Loading Operating system" after the initial post.   I again ran a hard drive test and the hard drive checked out fine.  It does not seem to be over-heating.

    Here is my question;  What caused the "Error Loading Operating system"?  Was it the wrong adapter suppling the wrong amps?  Would a bios update fix the problem?  I would like to know what caused the problem so I can prevent this from happening in the future.  

Answer : Error Loading Operating System

not sure it come from these causes. it could be a windows bootup problem. file missing (ntldr or so on).
did you try to reinstall windows ?