Thanks for your answer! I checked the mentioned directories but unfortunately I haven't found any big files in them. With Explorer I summed up the size of all files on the disk (184GB). According to Explorer the disk size is 287GB, 26 GB free so 77GB is "somewhere else". I asked my colleagues and it turned out that they had the same symptoms earlier, and had to reinstall Windows 7.
Is there any good disk manager program to analyze the disk or file system structure (I run the usual utilies but haven't found anything wrong.