absic,
So, the Sempron 145 arrived yesterday (a day late) and I dashed home to try it out.
Fitted the Sempron into the MB and switched on.......... and the system booted, Windows up and running, all is well, the theory appears confirmed.
Then I checked the System Information, which reported BIOS version FFa in use. A puzzle, and slightly disturbing - does this mean my FX-4100 is dead? Does this indicate that the BIOS swap failed, as I would have expected the backup to be FE at the latest?
Just in case I re-flashed the BIOS at FFa anyway, and replaced the Sempron with the FX.
Switched on - and all is fine again.
So, problem solved although it's not entirely clear why/how. Is it possible that it was only necessary to remove/replace the CPU to fix the problem?
One slight anomaly to note, when Windows booted the first time with the Sempron, it reported Found New Hardware "PCI Bridge" or similar. I don't know if that is relevant or not.
Anyway, many thanks for your help. When (if) I get a chance tonight I'll try the Alt-F12 procedure.
Regards,
BNP