Grow the file or add additional files on another drive if it filled as part of regular activity.

On SQL 2000 it's very hard to identify queries using TempDB, the DMVs added in 2005 make it a lot easier.

Look for queries/procedures with large temp tables and see if you can reduce or remove the temp table usage. Queries doing lots of sorts or hash joins/hash aggregates should be investigated, see if they can be tuned.