Well its not a security flaw. But you need to know about 'Zune Transcoded Files Cache'. Today I was cleaning up my Hard Disk (M/C has Vista) and while diggin in through the folders, I found an interesting folder -> C:\Users\B\AppData\Local\Microsoft\Zune\Transcoded Files Cache. This folder had around 2GB worth of '*.tmp' files. :P
More info about the 'Feature' is explained here. Also the cleanup on the same (not sure yet), is present in Disk Cleanup (Accessories\System Tools).
All I want to say is that you can safely reclaim this wasted space and keep doing so periodically.