I had been losing space on Drive C for awhile and it seemed odd to me as I rarely store things on Drive C, therefore the space should not be going down.
It ended up I had 255 GB (!) of files stacked up in the Windows Error Reporting Service queue in C:\programdata\microsoft\windows\wer\reportqueue
Definitely worth checking from time to time if you have apps that crash a lot and use Windows Vista.
To find out more about this subject, as well as instructions to disable it entirely, please see this link