I know disabling DEP is dangerous, I just tried it as a last resort.
I uninstalled my codecs a long time ago and started using VLC, but that didn't help. It would only help as a work-around until something else triggers the error. (What was described earlier.. having a few hours of good behavior followed by more crashing)
Last night I grew aware of something that might narrow this down. On top of right-clicking the desktop and recycling bin, I noticed that the crashes were more frequent if I had an external memory device plugged in.
This piqued my curiosity, so I went to go check out Device Manager.. and mmc.exe crashed, with ntdll.dll as the culprit. I can open the Microsoft Management Console itself.. but the crash only occurs if I try to choose the Device Manager or Services. Interesting.. Still happened after I ejected the storage device, and it doesn't happen all the time. Safe Mode at least guarantees access to mmc.exe.
A clean install would result in the same thing for my laptop. I have restore cds instead of an actual windows copy. As I install basic software (multimedia player, burning software) it's likely this will trigger again. It's like walking blindly into a field of landmines.
Hope it doesn't carry over when I get my Windows 7 upgrade in October