After formatting and installing Windows XP Home plus drivers, chipset, audio, graphic, and network it seemed like a logical time to install the latest free version of AVAST 6.0.2011.
Computer was successfully restarted at each of the above steps until I restarted after AVAST installation. A restart then gave me a BSOD with the following “STOP: C000021A {Fatal System Error} The windows login process system process terminated unexpectedly with a status of 0x00000080 (0x00000000 0x00000000)”
I thought memory failing but that tested ok. I thought hard drive and tried another with same error. Removed the modem and changed the 2 256mb RAM sticks. Again same error. Changed back to the first hard drive with the new RAM and modem out. Installation is smooth so far. I will try Version 6.0.1125 one last time but I will fully install ALL Windows updates before proceeding.
Been working on this for 3 full days. The updates should be completed soon and I will post what happens. I have nothing else to try.
Computer is a Dell Dimension 4550
When you installed XP was it XP SP3? If not, did you update XP to XP SP3 before attempting to install Avast?
If not then you might want to make sure that XP is fulling updated all through SP3 and beyond before trying to install Avast 6.x.
You can submit the file to the avast ftp site for analysis.
Sorry, I’m not in my personal computer right now and don’t have more info, but you can search the board for the ftp.
Problem Solved 
Nevisos, you are right, SP3 or maybe just SP2 must be installed before installing AVAST 6.0.1125.
I said in my first post “I will fully install ALL Windows updates before proceeding.” I did install all Windows updates before installing AVAST latest version. All is now well. Too bad I had to make so many XP installs. It seems there should be some warning about compatibility or at least Avast should detect if XP3 is installed and abort with a compatibility message. Hope someone is listening.