I need help in this regard, I have read so many forums and
article/books pages, but I could not find answer of this question.

There are two modes of account creation in sharepoint

1- Domain Account mode (where you have to create AD account first
and then shareping user using the web interface)

2- AD Account Creation Mode (this option is for ISP, who needs
Account creation only using sharepoint web interface, which creates
account first in AD and then in sharepoint)

By default when you installed SPS/WSS, it installed in Domain
Account Mode, we need to install in Account Creation Mode, all the
articles and forums has talked about that but no one mentioned how
to that.

After my R & D I found that, it is one time option and it can be
choosen at the time of configuration database but we installed SPS,
WSS again but no option was there.

Please, anyone can answer me in step by step format or if there is a
command using stsadm.exe then please mention that where we should
run that command.