The Half-Life 1 Sierra Utilities route, eh?
For anyone that doesn't know about it, the original Half-Life used Sierra Utilities to install/uninstall, and when you uninstalled it would delete everything from the root directory. If you never changed the path of the install it would be fine (like Program Files>Sierra>Half-Life) however if you changed it to lets say just have the Half-Life folder in the Program Files folder, it would delete every single file in Program Files.
I had this issue happen first hand way back when.