Following on from Andras post (and I agree - don't go changing the system tables) - what is it you're trying to do that you can't do with regular DDL? There's a good reason that you can't normally change the system tables in 2005 - it can cause nasty problems.
Paul Randal
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Blog:www.SQLskills.com/blogs/paul Twitter: @PaulRandal
SQL MVP, Microsoft RD, Contributing Editor of TechNet Magazine
Author of DBCC CHECKDB/repair (and other Storage Engine) code of SQL Server 2005