I thought I had a battery problem (the symtoms from someone else's problem seemed the same, and foolishly took out the coin sized battery from my desktop.
I was out for maybe a minute, but when I switched back on I started to get CMOS errors, and now the BIOS doesn't see any of the HDDs.

What can I do to recover.

Rannoch

you will get CMOS errors as when removing the battery you cleared the CMOS

have you reset the time/date etc?

in the BIOS are the drives set to auto detect?

I've reset time and date.
I've set drives to auto detect.
During the boot up, it tries to detect the drives, and it now sees the secondary master and slave, but not the primary master and slave.
Rannoch

check all your cables, when removing the battery you may have knocked them slightly

Update - I now see Primary Master, Secondary Master, and Secondary Slave (CD). The Primary Slave is where the operating is located.

Rannoch

I can now see all drives.
I've re-installed XP, which is working, and I have an Acronis backup.

The main concern now is that in order to see what's happening, each time I switch off the PC, I have to disconnect the power cable, and then re-connect. If I don't the monitor does not come on.

Anyway, thanks for your assistance.

Rannoch

I would check the BIOS

does the machine keep the time and date OK?

Yes, the date and time are OK.
Rannoch

you could try removing all the connections to the motherboard, power, IDE etc moving the jumper to clear CMOS for about 10 minutes and then connect it all up again, hopefully it will clear it and put the bIOS back to factory settings