I keep having the exact same issue in Windows 7 64 bit and it always resolves when I disable Dropox in msconfig.exe/startup.
Every time I use Dropbox it re-installs itself in startup so I have to go back in and uncheck it.

This is probably the wrong section for this question because it's not a BSOD and I can keep working but it does take an app or two down with it so I have to restart to load it/them as they only load on startup.