Windows XP no longer boots VB 5.1.x

[pquiring]

My VMs with Windows XP don't boot any more after I upgraded to VB 5.1.x (5.1.0 and 5.1.2).
If I downgrade back to 5.0.x it works fine.
My Win7 and Linux VMs boot fine in 5.1.x.

What is strange is that the BIOS doesn't even show.
Just a blank screen.
All of my WinXP VMs are a clone of one I created. Maybe some settings that got cloned is preventing the BIOS from booting.

Any ideas?

[pquiring]

If I create a new VM and just reuse the same vmdk file it works. I'll just recreate VMs for them all.

[mpack]

My XP VMs boot fine, most of which were created last decade.

If possible, do a comparison between the .vbox files, see if you can pin down the important difference. I'd start with the IO APIC setting, since setting that the wrong way (not how that XP was installed) is known to result in black screen.

[pquiring]

I know what it was. I was using a modified BIOS to run WinXP Mode which I guess the BIOS image is no longer compatible.

I'm upgrading all my images to Win7 since I have volume licenses for it.