My experience in installing Microsoft Security Essentials.

I tried to install MSE on my XP laptop and I got an installation error - 0x8004FF01.

After looking through a couple of posts I tried the following:

1. Download and install the latest Windows Installer 4.5 from here:  http://support.microsoft.com/kb/942288
2. Ensure the Windows Installer service is set to automatic and started.

After doing these 2 things, the install worked fine.