Windows only: Sweeping through your system and deleting unnecessary files feels great—until you realise you've just permanently wiped your master contact spreadsheet. Avoid that kind of over-zealous deletion with fsLock, a free Windows tray utility that prevents a list of files from being deleted, moved, renamed, modified, or otherwise touched. The utility has to be running to work, of course, and it used about 8K of memory on my Vista laptop, but I couldn't mess with my chosen files inside Windows. It's not a privacy or security tool, as you can get to the files through another OS or before booting, but it keeps your files safe from the casual destruction of "Select All->Delete." fsLock is a free download for Windows systems only.