I've written before on this site about programs that provide an on-screen hard disk indicator, which can be useful if you can't see the one on your computer itself.  Maybe the box is under your desk, or in a cupboard, or perhaps it's a virtual server in a datacentre hundreds of miles away.

Activity Indicator, which you can get from http://sourceforge.net/projects/activityindicat , is actually a very good program.  It's free, it's portable, it's free from malware according to VirusTotal and Web of Trust, and it provides a perfectly acceptable disk activity indicator on your system tray.  But look a little more closely and it also has another incredibly useful facility.  It logs every access to every file on your hard disk.  It can tell you each time a file was created, deleted, or replaced.  Not only is this a really useful troubleshooting tool, but it's also a great way to learn about what Windows gets up to when you're not at your keyboard.  And if you suspect that someone else is misusing your PC, this is the tool that will help you track them down.

If you support computers for friends, family or colleagues, this is something that really deserves a place in your toolbox.