Thanks all, especially to eidenk and dencorso for all their contributions to this thread.
As the thread starter, I'm sorry I haven't contributed to it recently, but I wasn't getting any e-mail notification of replies for some reason.
One development, that I have now noticed on my system, is that the boot problem actually seems to appear in two stages.
I was always getting the VFAT error BSOD on startup if system.dat got too big, and I've been trying to prune it down as much as possible, both by using automatic compacting programs, and by careful manual editing.
If the system.dat file size is below around 12.5MB the system boots fine.
If it gets above that, but still below about 12.7MB, the system doesn't blue screen on startup, but just stops on a flashing DOS cursor after the splash screen, and never goes any further.
Above about 12.7MB system.dat size, the VFAT error BSOD appears.
I don't know if this gives anyone any further clue as to what's actually happening here.
I'm still looking at removing all software that writes a lot of registry data, and which I don't use much, to work around the problem, and get the registry size down to a level where I don't have to keep worrying about the system failing to boot all the time!
I read that the .NET framework is a big registry data writer, and I have both v1.1 and v2 installed.
I still need 1.1 as I know that I have software which I don't want to uninstall which uses it.
I am thinking of removing v2 though, as I suspect that alone has written a lot of registry keys.
Does anyone know what sort of applications might need .NET v2 as opposed to v1.1?
Edited by Dave-H, 17 November 2007 - 05:54 AM.