The AR System database user (ARAdmin by default) and password
(AR#Admin# by default) are set during AR System server installation.
I have that same environment as you, if you look in Microsoft SQL Enterprise Manager, expand databases, expand AR System, select users and you can see it says DBO, Login Name ARAdmin.
User name ARAdmin
Pass AR#Adnin#
To test, register the server in SQL Enterprise Manager.
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Win2k3 Enterprise Server / SQL 2000 / Remedy 6.3 patch 24 / Websphere 6.0.1 Mid-Tier, Kinetic Request 4.0.4