Unable to unlock set up - password unknown

I guy in my neighbor is using Windows Xp Home Media Center SP2 system whose Antivirus was expired but he never bothered about the same to update or renew. Now he has got various Viruses in his system and his system administrator is also hi-jacked. He is now also not able to install or uninstall any application from his system. So he brought the PC to me.

I did a clean format and re-install Windows XP from scratch. Also installed McAfee antivirus, updated and scanned the complete system to get it cleaned. Everything was looking fine, I was able to install or remove applications. But when I tried accessing Setup, it was locked and asking for password. I enter the Admin password but it says it is Invalid password.

I even tried using Offline NT Password & Registry Editor boot disk but still unable to unlock the same. Can anyone of you please help me with this? Why is it not accepting the password?

Did you formatted the entire hard drive before installing XP Again? If not, I am 100 percent sure that computer is still not virus free. No matter you format C drive how many times on an infected computer, system wont get clean after running antivirus as well. So just tell that guy to back the very important only (as it may also have virus) and you just format the entire hard drive again, create partitions again and install XP again. This is called CLEAN INSTALL.

Nope, i dint formated the entire HDD, i just formatted C drive and installed XP again. But Thank you mate, I will try that as soon as I can.