One of our clients had viruses infect his XP computer, after removing the viruses from the computer, we then tried to install Microsoft Security Essentials. I would go through the setup wizard and when I went to install Microsoft Security Essentials I would get the following error:

An error has prevented the Security Essentials setup wizard from completing successfully. Please restart your computer and try again, Error Code: 0x80070780.”
Here are the steps I used to correct this error and then I could install Microsoft Security Essentials.
Step One: Browse to “c:program files/Microsoft Security Client” on your hard drive. Microsoft Security Essentials is trying to install in the Microsoft Security Client folder but the malware has corrupted the folder.