You're not the only one having problems with the Den. I'm also using the 1.04 patch, and am assuming it's likely the culprit. Every time (past the first) that I try to enter the west side of the Den, the game crashes to the desktop with an illegal operation error, with Kernel32.dll having some problem or another (what is it with Kernel32.dll anyways ? It's always screwing up). Thus far, the only problem I've noticed with the patch, but it's more than annoying, as this cuts me off from getting the car. My assumption is that it has something to do with the fact that "Brotherhood Bases in the Den and NCR are now equipped," as the readme puts it. Also noticing a few things that were supposed to change in the patch didn't, such as "Beer now weights 1 pound."
I've just spotted that, for some reason, I'm back down to DX6.1. Going to fix that later and see if it makes any difference.
In case it's relevent, and due to the sticky post, here are some of my sys specs:
Win98 SE
DX6.1 (?)
Athlon XP 1700
384 MB DDR
Geforce 2 MX 200 (my GF4 died...)
Also, I'm on a day old reinstall of Windows (I was having the same problem before the reinstall), the drives are defragged, etc. Definately something faulty in the patch.