Do not change the system table by adding a trigger. This is a very bad idea. A trigger that doesn't work correctly on a system table could take down your whole system!!!!! If you must have this functionality, upgrade to 2005 or 2008.
Personally, I think you would be better served by taking away all access to production tables except through a dba or team of dbas. No one should be making structural changes to your prodcution database except through a script. If no one has the rights except the people who are trusted to do it right (and who can be held responsible for what was done), then you don't need a trigger to know who made a change, you already know who has the access to make a change.
"NOTHING is more important in a database than integrity." ESquared