has your free trial ran out you may need to crack it have you had it for more than 120days ?
use this http://www.lomcn.co.uk/forum/showthread.php?t=54059
hope this helps you
Get SQL Express its free, wont run out and should handle mir
Run the install again, and then select then repair option.
Run Setup -> SQL Server 2000 Components -> Install Database Server -> Next -> Next -> Advanced Options -> Registry Rebuild
That should sort your problem.