clustering an existing instance of sql 2000

We have a client running a 3rd party app on sql 2000 Enterprise on Windows 2003 Advanced attached to a new SAN. They are looking at adding another SQL server and clustering for failover. Anyone done this before. Do you have to reninstall sql server on the original box to do this. This a a heavily used app, so downtime is always an issue.

You can build a new sql cluster then move the db and app over.

Are you saying create a cluster on the 2nd server and then add the old one?

No, you need two servers to build cluster. You can use existing server, but expect down time during cluster build.