The other day I had to install a program and having a small primary hard driver decided to go for some cleanup... After asking Vista for the amount of "wasted" space I was puzzled:

Freeing 36.7 GB out of a 63.7 GB hard driver would have been great, but I was somewhat suspiciour to have collected 15 GB of reports and 21 of temporary files. In fact, after the cleanup, the system had the following status:

So it did free a lot of space (9GB) but far less than Windows Vista computed. How could it be so wrong? The temp folder has so many file (and it takes Vista minutes to sum them)... it is unbelievable they did the wrong sum! Hope Windows 7 is really goign to fix all Vista bugs, that would have been good enough for me to upgrade. (Ballmer, Vista is a quite crappy and companies are not using it, this is the real world... your fantasy world mgiht be different, though.)