When replacing certain hardware (e.g. motherboard, CPU), always perform a clean installation of the OS.
There's probably not too many users in my situation, ie. upgrade in hardware and still running XP.
I thought about what you said and installed a fresh copy of XP on an old HDD I had laying around
(Maxtor 10 GB UDMA33 drive from 2000). It took a while but it worked perfectly.
For my two live systems I decided to do the repair install available when booting from the XP CD.
I can report that this also fixed the problem. No more crashes in avast, Firefox, etc, etc.
Thanks Eddy