The registry is your PC's repository for all the important Windows settings for your hardware, software and system configurations. It's also the home to for all the applications to store their launch settings and other data.
After a time, invalid or obsolete information starts to build up in the registry. This is the the result if installs and delete of programs, creating shortcuts, changing systems settings, etc... etc... this happen has a natural part of using a computer and doesn't (usually_ have a major impact on your PC'S performance. However, it can cause system and program errors. The solution is to scan your registry periodically and delete invalid entries.
Vista comes with a Registry Editor but it is intended for editing only and not clean up. There are dozens of free registry applications to choose form n the web, I rather like Registry Mechanic.
Registry Mechanic offers a few different options like a full or selective scan, back up and the creation of a System Restore point. The last is CRITICAL and you should always create a restore point before you start deleting items from your registry! The trial version is "time unlimited and removes problems found in 6 sections of the registry only. A registered version is required to remove all detected problems."
If any one else has an alternate application, please share! 
I recommend cleaning your registry at least once a month. Doing so will keep your system more stable and therefore more reliable!
Learn here how to
create your own restore point before making any changes to your system's registry. This step I highly recommend before making and changes.
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