Your post is ambiguous. I can't tell if you have set a BIOS password or one
in Windows. If this is a Windows password, see below. If a BIOS password,
post back with whether this is a desktop or a laptop and if a laptop, the
make and model.

If you have forgotten your password, if you have another user account with
administrative privileges you can log into that account and change your
original user account's password from the User Accounts applet in Control
Panel. If you don't have another account like this set up or don't have the
password to it, you'll need to log into the built-in Administrator account.
In XP Home, boot the computer into Safe Mode. Do this by repeatedly tapping
the F8 key as the computer is starting up. This will get you to the right
menu. Navigate using your Up arrow key; the mouse will not work here. Once
in Safe Mode, you will see the normally hidden Administrator account. The
default password is a blank.

In XP Pro, you do not need to go into Safe Mode. At the Welcome Screen, do
Ctrl-Alt-Del twice to get the classic Windows logon box. Type in
"Administrator" and whatever password you assigned when you set up Windows.

If you reset the built-in Administrator account's password in Home or have
Pro and don't remember the password, use NTpasswd to change the built-in
Administrator account's password to a blank.

http://home.eunet.no/~pnordahl/ntpasswd/

Then go to the User Accounts applet in Control Panel and set passwords that
you will remember and make other desired changes. WRITE THE PASSWORDS DOWN
AND PUT THEM SOMEWHERE YOU WON'T LOSE THEM.

Malke

You don't "reinstall" an old computer.  You can rebuild an old computer
but it's likely that's not what you meant.  You reinstall software, like
an operating system or application.  If you are reinstalling the OS, you
don't care what was the old password.  You'll make up a new password for
the newly installed OS.