That can't be caused by anything you did in Windows, because the PC should boot from a bootable Windows XP disk even with the hard disk totally new and blank and empty. It boots the OS from the CD, only then starts to look at the hard disk and see what it can do with it.
It's not clear from your post what you did after searching for the update (and maybe finding it?) and receiving the BSOD. Did you download something, install something? Or did the BSOD come before you even started the download. You simply don't tell and leave us guessing.
If the boot device is set to cd first AND the xp cd is bootable (as an authentic Microsoft CD is) and it still doesn't boot, you can be sure it's a hardware issue somewhere that needs repair.
You can check the boot device in the BIOS, as you say. You can check the CD on any other computer where you can access the BIOS. After those 2 simple checks you know more.
I've got a Dell 8300, P4-2.80G, 1G DDR SDRAM, XP Pro. I made the cardinal error of trying to fix something that wasn't broke. I decided to see if there was a driver update for my Maxtor HD, using the update driver function in the Device Manager. It searched for the update and then I received a BSOD with this error ?Stop: c0000221 Unknown Hard Error \SystemRoot\System32\ntdll.dll?. I shut down the system and rebooted. I get a momentary manufacturers splash screen. My two hot keys are F2 and F12. I can reach the BIOS information and the Boot Device Menu, that's it. I can't reach "Safe Mode", I can't reach anything. I used my XP Pro disk and the system won't accept it, and it's the disk that came with the system. I got a new copy of the ntdll.dll and tried to replace it but I couldn't do anything with it. I consider myself fairly knowledgeable but this has left me with the taste of rank amateur in my mouth. Any ideas? Please?