Crash on startup between agpcpq.sys and acpi.sys -- Win XP

My Dell Latitude D505 laptop is not booting. In normal mode, the Windows XP startup screen displays, and then goes blank and hangs indefinitely. In safe mode, it lists drivers up to agpCPQ.sys, and hangs. The next driver to load would have been ACPI. ("Did not load driver Advanced Configuration and Power Interface (ACPI) PC")

I do not have the XP startup/installation disc(s). In my frustration I may have taken a couple of steps to worsen the situation. Unless otherwise noted, everything I tried was performed in the mini-xp environment included in Hiren's Boot Disc. Here's what I've done so far:

Believing it a video problem, I tried to install the most recent graphics drivers, R111509.EXE and then, when that failed, R78792.EXE, which both hang with the message "Installing version...," but when I try a second time, it complains that they're already installed.

I restored the registry from a backup that I had made, to no avail, and then restored the registry from a backup from when system was first installed, which also flopped.

Booted from XP recovery console CD and ran chkdsk mixing and matching various flags, ran fixboot, ran fixmbr, tried different boot.ini configurations.

Ran rootkit removal tools and other malware/spyware tools with latest updates.

Upgraded to the most recent BIOS.

Disabled acpi.sys (which was next in line to load during the crash), and got a BSOD reading: STOP: 0X0000007B (0xF78C6528,0xC0000034,0x00000000,0x00000000)

Held the fn key and booted without the battery, performing Dell's pre-boot diagnostic. Everything passed.

Reset the CMOS battery.

The last thing I'd done (before the problem arose) was uninstall Windows Defender using iObit Uninstaller software. Perhaps a lousy idea, but I'd read you can uninstall it and use whatever other program(s) for antivirus/firewall, which I'd already been using.

I'm out of ideas.

Oh -- one more: Is it ok that (what I think is) the boot partition is in FAT format? I think that's what I'm seeing (copied below):

diskpart returns, 57232 MB Disk 0 at Id0 on bus 0 on atapi [MBR] -: Partition1 [FAT] 47 MB < 39 MB free> C: Partition2 [NTFS] 57159 MB < 36603 MB free>

Is there an easy way to change that without formating the partition? Or would that even be necessary or worhtwhile (or safe, eeee-VUHN)?

Anyway, I think I've exhausted my imagination as to what the problem could be, and turn to the internet for guidance. I imagine it's a lost cause, and even so, I'd love to hear at least how people would find the problem, even if it can't be fixed without reinstall. I'd like to know how to hunt that sort of thing down, being new to Windows. After dozens of Google searches, much learning and much frustration, I throw in the towel and humbly enquire: what to do?

I got mine going uninstalling all of the anti-virus (BitDefender free and Malware Bytes anti malware). It is possible an anti-virus update caused some kind of internal malfunction. This last thing SEEMED to have fixed it, but I did re-install Windows as a last result just previous to this.

If uninstalling anti-virus doesn't work, do try an "in-place install" / re-install. One or both of these fixed my Win XP, but I am not sure which.

I'm currently fighting with Genuine Validation as it has asked me to re-enter the key, and now won't accept it as genuine. This is a separate issue, however.