gaining COA

first of all, I can't logon to my computer. I had just
bought my computer from a friend who had a weird
password. I changed it to my wifes name for
administrator. now i can't logon though.

would i be able to fix the logon on problem through re-
installation of XP. cause i got a DELL pc that also has a
reinstallation cd for XP?

But, I'll also need the COA. would anybody know how i
could get that.

for additional info, i also have the pc on a work group
with a pc with Win 98se. but, i can't log on through
there either. first of all, cause i don't know how to.

please help me gain some type of help.

thank you