Splunk should install fine on Windows 2000. The only that we know for sure doesn't work is the monitoring of the Registry.
We don't actually support Windows 2000. It is known that WMI in particular does not run well on that OS. If you want to get logs from these boxes, your best bet would be to install Splunk forwarders on the 2000 boxes, use the native event log collection method and have the logs forwarded to an indexer.