Something to watch. If you originally installed SQL 2000 to a directory such as C:\MSSQL but moved the errorlog to say E:\logs by changing the switch in EM, when you install SP1 the location resets to the original location. i.e. C:\MSSQL\LOG
If this directory doesn't exist then SQL will not start after installing SP1.
I have told Microsoft about this gotcha and they have confirmed it.