If u have a large custom content collection try moving it out of "my games" folder completely temporarily. You dont have to bin it just move it elsewhere on the hard drive. Then try and launch UT3. If it runs its in your custom content. You would then put the custom content back in and remove it one piece at a time and try to launch it. Cut it in about 4 pieces to start with. I just use gametype coz i have subfolders for them but you could use a quarter of the alphabet. Once you find your faulty piece. Subdivide it in half again and test. Repeat process and you will find the prob. It may take a while if you have a large collection, but by halving the search each time it wont be too hard. Ive done this with several hundred maps on my comp. If you have mutators and maps you may want to start off by removing one then the other to start your search.
From my experience if your UT3 crashes from a fresh re-install its a buggy content problem. You could also delete your "published" shader cache's as well, sometimes that brings it good too. (although your maps are slow to load first time you play them again while it rebuilds cache of course.)
Anyhow hopefully some of that helps in your diagnosis
[EDIT] If you can remem the stuff that u added at the time that it stopped working start by trying without that in. When I get a heap of new maps now I keep my backup copy in a diff place, from tested ones now. This helps heaps when this happens, coz u can go back and check off against the list and limit it to what you downloaded. If you dont keep backup copies for DVD backup Id prob create an text doc of what your putting in at the time, or just use the download list on firefox (and remem not to clear it when you grab a shedload of maps lol
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