Any time you change settings or delete items from the registry you have the possibility of something going wrong and turning your computer into an excellent paper weight.
No the registry does not clean itself.
There is a free registry cleaner that is recommended by many, but again use all registry cleaners with caution.http://majorgeeks.com/EasyCleaner_d414.html
How to Backup the registry:
Start>Run> (type) regedit
Backup the registry by:
1 - Highlight My Computer by clicking on it once.
2 - Click File (Windows XP) or Registry (Windows 2000), then Export
3 - Select export range All; to ensure your entire registry is backed up
4 - Select where you would like to save this backup, name the File, and
Click on the SAVE button.