Quick Registration

[dotagious]

I want my existing registration form to only include:

username, password, email, and the image verification field.

There are two quick registration hacks, but one ADDS a quick registration option and the other one inlcudes too many fields and excludes the new image verification feature.

The thing is... I just want to change my existing form to include the fields explained above. What template(s) and/or php file(s) would I modify?