I want to replace winnt.bmp with my own so it shows when XP is loading.
While XP is loading is a bit tricky. After XP loads but while nobody is
logged in / while you are logging in is a bit easier, and is what you
appear to have been attempting.
So, do this. From a command prompt, type the following and press enter:
reg delete "HKU\.DEFAULT\Control Panel\Desktop\Wallpaper"
That will remove the mistake you made.
Then type the following and press Enter:
reg add "HKU\.DEFAULT\Control Panel\Desktop" /v Wallpaper /d "C:\MASM32\SOURCE\XP_Startup_Pic.bmp"
That will make it so that C:\MASM32\SOURCE\XP_Startup_Pic.bmp is
displayed as soon as the window manager loads and can display a
graphic as a background / wallpaper.
Then, type the following and press enter:
reg query "HKU\.DEFAULT\Control Panel\Desktop"
That will allow you to verify there is an entry that looks like this:
Wallpaper REG_SZ C:\MASM32\SOURCE\XP_Startup_Pic.bmp
If so, reboot and it should work.
If not, run the command a gain and post the results here along with
a careful, detailed description of what you see during the startup.
If you get any errors during any of the above steps, stop, and
copy/paste the command that generated the error along with the
text of the error into your next post.