Starting an App during the welcome screen windows XP

A third-party app that can accomplish this too,
http://www.firedaemon.com/

As noted in the bleeping computer link provided above, services start before the logon, so IF a program can be run as a service you can ensure it will run at the welcome screen before the user logs in.

Otherwise, there's little control over exactly when startup programs get control -- you're likely to already be at the desktop when they run ... that's why it takes a fair number of seconds after the desktop appears before Windows is really "ready" for use.

is it possible to run an application over the logonUI.exe ?

Not sure what you mean -- anything that runs as a service will run before a user logs in.   Are you looking for a way to run something concurrent with the login? ... or instead of the login?

I'd be looking for something which runs instead of the login.

as long as it doesn't interact with the desktop then it is possible..
put your program in the
HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\bootexecute

But doing that won't allow me to run a program over the top of the logon process will it ? So essentially as the welcome screen shows or before it shows my application kicks in and runs while the logon process is happening in the background. Not sure if that is possible. ?

No it won't allow you to run a program over the top of the logon process.  Could you give me an idea of what you are trying to accomplish?  Bypassing the logon process brings up the spectre of breaking windows security.

If you simply want to run something before logon, run it as a service (as I noted above).

If your goal is to run it INSTEAD of the logon, then you could instead simply make a separate account, set it to auto-logon; and run the program you need as a Startup program.    Your program would then have control and could do whatever it is you need to do.

Yeah, my plan didn't work very well so I've decided to run the program as a service.

What I wanted to do is I'm repurposing some old IT equipment and turning them into thin clients. I have created a RDP application with vb.net with it's own customized logon screen logo etc. I wanted to see if it would run over the logon screen /welcome screen so no one knows I'm using a Windows XP Operating system.

customize the boot screen, have the machine autologin and replace shell=explorer.exe with
shell=mstc.exe  or your custom application

stardock bootskin is handy to customize the boot screen

Thank you, ve3ofa :)

Until I had an idea of what you ultimately were trying to do .. and what you can do without breaking the o/s .. I was starting to get bad vibes.. as to why someone would want to do what you desired.  Now instead of trying to get around the system you are working with the system.. better all round.  Thanks for sharing your ultimate goal.