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The xoops_users table contains that information, but the password is one-way encoded.
You could try the "Lost Password?" function, which will generate a new password and send it to the email address you registered with.
If that doesn't work, you could use this query to get the username for user #1:
SELECT uname FROM xoops_users WHERE uid = 1
and this query to reset the password:
UPDATE xoops_users SET pass = MD5('whatever') WHERE uid = 1