Finally got another SSD fitted, and I've managed to install Windows 8 from my 64bit disk.
It allowed the install to complete fine, but won't let me activate it as I've already used the serial number on my netbook.
I've now bought another cheap Windows 8 serial number on-line, I hope not from anywhere dodgy!
They use PayPal, and look legit, but I suppose they always do!
I'm still waiting for it to be e-mailed to me, but if it all goes wrong at least it was only $40.....
Once I've got it activated, one way or another, I will do the free upgrade to Windows 8.1.
It all looks very hopeful, all the hardware was installed OK, and it looks as if I needn't have worried about the NX bit!
At the moment the only way I can switch between booting Windows 98/XP and Windows 8 is by changing the first boot drive in the BIOS, which is not terribly convenient, so once I've got Windows 8.1 up to speed I'll look into a multi-boot interface of some sort.
I used Easy BCD on my netbook to give me the actually rather elegant Windows 8 OS choice screen so I can choose to boot to Windows 8.1 or XP.
If I choose XP the machine completely re-boots into XP, but I gather that's how it has to work.
If I can get that to work on my main machine I'll be very pleased, but I bet Easy BCD won't know anything about Windows 98!