I don't suppose you remember what you deleted in the registry?
I do, to some degree. I did take the precaution of making screenshots of everything - and guess where those are? Anyway, here's what happened: Found a forum while looking for reasons my CD/DVD drives were hosed. In one forum, there was a post that gave step by step instructions for restoring a CD drive, from a guy who'd found an answer. There were 10+ enthusiastic kudos for his suggestion. Impressive. (The name of the forum starts with the word "Tool" I think. I looked for it for more than an hour last night, but no luck. I did bookmark it at the time, but you know where that is.)
Here's what he wrote, basically:
"Open regedit, find HKEY_LOCAL_MACHINE\System\Current Control Set ..." That's as far back as I remember with respect to location. I do remember, however, that I deleted the names "Upper Fields" and "Lower Fields", per his post. Those came under one of the long alphanumeric entries with an "e325 11ce" in the middle, if that helps. I reference his post on the chance that someone may have seen it, not to try & subtly shift responsibility for the screw-up. I'm the idiot who can take credit for that.
Now, according to this fellow's well-received instructions, that should have fixed the CD drives problem. I rebooted (I think), but Windows still didn't see the drives. OK, back to the forum. Read the instructions again, taking screenshots. Decided maybe I missed something and re-entered regedit again. I found, within the same area of the reg. another couple of "Upper" and "Lower Fields", but this time, instead of deleting, I renamed them to "Rename" and "Renamed", so I could find them easily and edit back to the original name. Rebooted, and that's when the mouse/KB wouldn't work once XP began the boot process.
In the BIOS setup (F2 on a Dell), there's an entry called "System Log" or a similar term. I hit Enter and read the entry for that day and it reported a keyboard failure. I knew the KB worked, so I chalked it up to the source of the main problem. An effect, rather than a cause. I hope this is all of some help, and believe me, your interest is sincerely appreciated.
If you are using a USB mouse and keyboard, try switching to a PS/2 mouse. On some systems, changing the boot order in the BIOS setup allows a USB mouse to work (i had a Gateway PC with that quirk).
I tried switching from a PS2 mouse to a USB mouse, but it didn't help. And I also tried different boot orders. But since the CD drive isn't recognized, and I didn't have a floppy to boot from, putting those two at the top of the order made no difference, since the system saw the HDD as the only viable boot device. Good idea, though and thanks.