The stored procedure "master.sys.xp_instance_regwrite" is not documented in SQL Server Books Online (BOL), but is usefull. In general the syntax is

EXECUTE [master].[sys].[xp_instance_regwrite]

  @rootkey = N'<name of root key>'

 ,@key = N'<key name>'

 ,@value_name = N'<key name value>'

 ,@type = N'<key type>'

 ,@value = <key value>;

For example the number of SQL Server Errorlog files can be updated by the statement

EXECUTE [master].[sys].[xp_instance_regwrite]

  @rootkey = N'HKEY_LOCAL_MACHINE'

 ,@key = N'Software\Microsoft\MSSQLServer\MSSQLServer'

 ,@value_name = N'NumErrorLogs'

 ,@type = N'REG_DWORD'

 ,@value = 42;

Use with caution!

The parameter @type takes a value without N'' around (@type = <key type>), but I like the Unicode framing. This way I try to avoid difficulties when building the values outside the call.