Does it still happen after a clean install
with no 3rd-party plugins?
Post a list of all plugins (including defaults) c/o a my_plugins_list.txt as generated by DrO's gen_nunzio plugin (see here
The same thing used to happen to me quite a bit, especially when an Explorer window was open which contained the files I was editing at the time (damn WinXP and it's Tiles/Info views). But it hasn't happened to me since I disabled a few 3rd-party plugins I never used, and updated a few others.