The Zabbix tool, posted in the comment, is not really working, because the server doesn't run on Windows, only the agent. See info in their requirements page. This means you'd need to install the tool on a Linux machine and only an agent on the Windows machine. Great if you have a heterogeneous environment, not really helpful in a full Win environment.
The same situation is for Nagios, another interesting open source monitoring tool. They say there is a package only for Windows, but the doc page it redirects to doesn't work now.
Of complete free monitoring tools useful for SQL 2000 I'm not really aware, but you can use successfully a pair of:
Perfmon (find more info here)
Server trace (trace specific queries, not just everything) -> actually files that can be open in SQL Profiler
SQL Alerts (info in MSDN article or here, in this SimpleTalk article)
PS: found that a great monitoring tool is also working on SQL 2000. So you'd better try to test Confio Ignite free. It's fantastic for the price :-).
PS2: there is also a small free monitoring tool from Idera -> SQL Check, but I don't know if it works on SQL 2000.