If you want to change a SQL Server service account, you should use SQL
Server Configuration Manager.

There are BUILT-IN SQL Server local groups which has enough privileges to
run. When you change your service accounts related to SQL Server using SQL
Server Configuration Manager, the new service account is added to those
related groups automatically. So SQL Server would run with that account
happily.

Hi

For SQL 2000 the service account can be set through Enterprise Manager. You
may want to look at http://msdn2.microsoft.com/en-us/library/aa302434.aspx 
and http://support.microsoft.com/kb/283811

John