As you already suspected, In SQL2000, there's not much you can do.
If all your databases have the database recovery model set to "FULL" instead of "SIMPLE", you could use a log reading program to find anything that had changes in each database.
I'm sure the logs would not have any info from where people did simple SELECT statements and then stopped using the database. that's not logged.
Otherwise, you'd need to start running a trace to capture login information , so you could capture it going forward, but not based on past data.
In 2005, there's a lot you can do with triggers and auditing.
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