Once, on a different computer, I came home one day to find that my system32 files had been corrupted. There was no way to retrieve the files. I had to format that stuff and reinstall windows.
This ended up being a plus because it got rid of some stupid program on my computer which was installed when we got it called PCAngel which basically kept me from doing anything from transferring mp3s to my mp3 player to installing add-ons onto blockland.
Also on that same computer I ran TF2 for 5 minutes before getting a non-fatal bluescreen.