Specs:
CPU: Intel Pentium 4 531
RAM: 1.00GB Dual-Channel DDR2 @ 265MH
Motherboard: Dell Inc. 0DD332 (Microprocessor)
Graphics: Intel 82915G/GV/910GL Express Chipset Family (Dell)
HDD: 74GB Western Digital WDC WD800JD-75MSA3 (SATA)
Optical: SONY CDRWDVD CRX310EE
Audio: SigmaTel High Definition Audio CODEC
Green = No Issues
Orange = Minor issues that don't prevent upgrading
Black = Blocked from future upgrades.
Red = Major issues that prevent upgrading
Windows XP > Longhorn 3683
Upgrading to 3683 was almost flawless. The other issue that was encountered was when copying files error during the first part of setup.

This error did not occur when attempting the upgrading in VMWare Workstation 16 (Likely down to what I had installed on the Dell?). Interestingly, the file name always starts with the letter 'a' followed by a random string of 7 alphanumerical characters. Thankfully, the fix was to use the expand command on any file in the 'C:\$WIN_NT$~BT' folder to fill in for the missing file. (In the case of my upgrade, I just used CDFS.SY_)
You must ensure this file exists in the correct location, else when you reboot, setup will fail, saying that the file does not exist.
Longhorn 3683 > Longhorn 3706/3713/3718
Copying over files in the first part of setup is successful, but after rebooting there is a boot loop.
Longhorn 3683 > Longhorn 4001
No issues upgrading to Build 4001. Made a new user account in OOBE, had to transfer files over from the 'Owner' account made in XP.
Note!: Thanks to some research done by yourepicfailure, this technically isn't a true upgrade, as only Program Files and the Users folder stay where they are, the Windows folder doesn't get upgraded. They dubbed it an improper 'clean install'
Longhorn 4001 > Longhorn 4002
Launching setup from both the root of the drive and i386, both cause a 'catastrophic failure' and rolled back the changes before the first reboot.
Longhorn 4001 > Longhorn 4005
No issues upgrading to Build 4005.
Longhorn 4005 > Longhorn 4008
Marking this as a success as the upgrade itself was successful. However, the theme service fails to launch, attempting to start the service causes "Error 127: The specified procedure could not be found"

Longhorn 4008 > Longhorn 4011
No issues upgrading to Build 4011. Before logging in after OOBE, it told me my password will expire in 6 days.
Longhorn 4011 > Longhorn 4015
After the first reboot. I was greeted with a 0x6B BSOD, preventing me from upgrading to 4015.
Longhorn 4011 > Longhorn 4017
No issues upgrading to Build 4017. Again, was told that my password would expire in 7 days.
Longhorn 4020 > Longhorn 4029
The only minor issue to occur during the upgrade was when logging in after OOBE. To fix, login with the Administrator account and reset the main account's password as one is applied during the upgrade.
Longhorn 4029 > Longhorn 4030
No issues upgrading to Build 4030
Longhorn 4030 > Longhorn 4031
No issues upgrading to Build 4031
Longhorn 4031 > Longhorn 4032
No issues upgrading to Build 4032
Longhorn 4032 > Longhorn 4033 (main)
No issues upgrading to Build 4033 (main)
Longhorn 4033 (main) > Longhorn 4038/4039/4040/4042
The first part of setup is a success. However, upon first reboot "windows\system32\config\system" cannot be found.
Longhorn 4033 (main) > Longhorn 4050/4051
The leaked builds are PDC releases are unable to be upgraded to.
Longhorn 4033 (main) > Longhorn 4053 (1)


An unknown error occurred after the first reboot. I then restarted the machine and then produced the WinSetup.dll error.
Here's the desktop of 4053 with no USB support.

Setup then restarted and placed me into Windows. However, the drivers for the mouse and keyboard were not installed.
Windows PE works flawlessly though with the mouse and keyboard working.
Longhorn 4033 (main) > Longhorn 4074
Upgrades to 4074 are disabled.
Longhorn 4033 (main) > Longhorn 4093
Upgrade stopped by "Upgrades between componentized and non componentized installations are not allowed"
Updates:
(16/02/21) - I've ordered a PS/2 Keyboard so I will be retrying 4053 in the upcoming 2 weeks. (1) + Fixing lots of small grammar mistakes.
(17/02/21) - Added video + More grammar fixes. Added in screenshots for 4053
(18/02/21) - 4053 PE works fine.
(19/02/21) - Added poll to see if people want me to do this with other betas. I forgot this PC doesn't have PS/2 so rip that idea.
(03/03/21) - 4001 doesn't get fully upgraded. But we'll roll with it.