When installing Microsoft Security Essentials on Windows XP, you get:
Error code 0x8004FF56
Error code 0x8004FF56 occurs when you try to install Microsoft Security Essentials on a Windows XP SP2 PC without the Forefront Client Security Filter Manager QFE for Windows XP/SP2 installed.
To fix error 0x8004FF56, download and install the Forefront Client Security Filter Manager QFE for Windows XP/SP2 before rebooting then installing Microsoft Security Essentials.
❗ Note: Support for Windows XP ended on the 8th of April, which means that Microsoft are no longer releasing patches for Windows XP. This means that any new vulnerabilities discovered or released after this date will not be fixable by any antivirus program. You will get viruses and you won’t be able to detect or stop them. Therefore, in order to remain secure and virus free, I recommend that you upgrade from Windows XP to a supported operating system asap.