Computer help forum: System doesn't continue when starting windows

I had a running windows2000 system (latest updates) on my MS8147C+ motherboard.
Because i wanted to test if another AMD Athlon XP2600 still was OK I exchanged the one I had with the one of a non-working system . (Later I used that same processor in another system and all OK .)
But after exchanging the processors PC booted but when "Starting windows" appears it continued 'growing the white line' until 4 black blocks have not been filled and system hangs. On that same sytem I have a parallel installation (only for backup and restore) on another partition so I started that system but now the white line fills completely but still not going any further. This can be repeated.

So I did all kind of things to get things working again.
First (of course) i installed the original processor (STRANGE to me it doesn't work!!) . After that I echanged memory, video card (also AGP-card exchanged for PCI-card), pressed chips on the board, felt if mainboard became warm, exchanged dvddrives, exchanged IDEcables. I even removed my harddisk and added a NEW (still sealed!) harddisk and tried to install Win2K from the CD I've been using dozens of times. But Even while installing from CD system doesn't get further (and hangs) when "Starting windows 2000" appears while setup (so : all files have been loaded into memory).

I also installed windows98. That DOES work and system WILL boot and windows shows up. If I install win2K from Win98 upgrade it starts and also hangs (after restarting) showing "starting windows 2000" at setup.

I also flashed bios (with a newer one), still same result.

I checked the original harddisk on another PC : no virusses, no damaged sectors etc.

ANY explanation, suggestion or help to findout the cause of the problem and to make PC running again will be greatly appreciated.
I've bene installing and repairing installions hundreds of times before but never seen anything I encountered now.