GhostBuster is a powerful tool which will scan your Registry for ghosted devices (hardware which was once connected to your PC, but is no longer present), highlight these and allow you to remove them at a click.
Why bother? One reason is to clean up your Registry. There's really no reason for you to still have a Registry reference for a mouse which you broke and threw away two years ago, for instance - GhostBuster can delete this for you.
And the process can also be useful for troubleshooting. When a device isn't working then sometimes you can have problems getting rid of the old drivers, so that you'll be prompted to install new versions. GhostBuster may be able to clean away the old references, allowing you to install the latest drivers and hopefully get everything working again.
Please note, though, GhostBuster's actions can cause significant problems. If the program accidentally removes something important then you may not be able to use your PC until you've reinstalled the appropriate drivers. Make sure you a) check the option to create a system restore point before you delete anything, b) have your Windows disc or other bootable CD/DVD to hand in case something goes seriously wrong, and c) have a full system backup available as well, "just in case".
This really is just to make sure you're covered, though, as for the most part GhostBuster runs very well indeed. We used the program on a test PC where it reported 164 devices out of 395 were "ghosted"; we checked "Create System Restore Point", chose most of our leftover devices via the right-click menu, clicked "Remove Ghosts" and they were all deleted in moments, with no ill effects at all.
Note that this is the portable version of GhostBuster.