Apparently Notepad++ was the culprit. I uninstalled it and everything went back to normal. It seems that in order to draw its icon on the menu, Notepad++ forces Windows to revert back to its old method of drawing the context menu...
Still a bit weird no one here encounters the same problem though
*Edit: On a side note... only Notepad++ 5.5.1 (newest version) causes this weird behavior. Older versions are OK. Maybe I need to reconsider my habit of keeping my software up-to-date