hey, this is really stupid question.
But, when I try to set up the "server extensions web", the prompt pops up for the username and password for my computer, which I do not remember.

prompt: Enter username and password for """ at http://rem.
(rem is my computer name)
How do you get the username and password from command prompt? or in any ways?

Well first i would try hitting enter without anything in the fields being as you may not have made a password. Then try just "rem" as username and hit enter. Now if that goes nowhere, which i think it will :P. Google "recovering lost password on windows xp" or something similar.

if you are logged in as admin
just go to users and groups mmc and change the password
thats right click my computer manage usr groups find admin and fix
if you are not admin your talking hack passwords there a few sites out there that use linux boot disc to erase and build new sam file for password