A debugger for .msi is overkill...
Also you would have to break in msiexec.exe (the Installshield executable
that runs the .msi) It's a mess!

Alot easier way is to use ORCA (msi tables editor available with the MS Platform SDK)

The SDK is available for free from Microsoft but it is unfortunately a mammoth download.
So...
Get ORCA here:
/h--p://www.patchlink.com/downloads/support/orca.msi

Install it and use it to open your .msi
Now on the left side you'll see all the installer tables.
Locate your username/password dialog and look at the conditions for next/ok button.

Because there are a lot of tables in an .msi, focus on "ControlCondition", "ControlEvent", "Property" and "CustomAction" tables.
Generally look around in there and you'll soon get the meaning.

Post with more info (maybe even the program you're trying to crack) if you
get in trouble and I'll try to give more details...

PS. You were on the right path with your first post.
Don't stray from that! Just look at your conditions a little better and you'll get it.
It doesn't matter that the verification is done by an external dll, this is then
evaluated by the installshield script. If you find where you can tell it to be
happy with the result you're OK.

Hope it helps.