When data changes i.e. you delete a file, the registry doesn't remove the invalid data, thus in this case a reference to a non-existent file remains in the registry. As the registry gets filled with invalid data, Windows and programs consume more system resources to search the database for any data. Therefore your computer gets slower and slower.
- Viruses: Many viruses attack through the registry by creating or modifying keys. Unless you have a thorough knowledge of your system and registry keys or specific direction from a technical expert on how to search for these infected keys, it is very dangerous to try to edit the registry on your own. Any damage done to the registry at this point would require either a re installation of the registry from backup or a re installation of Windows. Either way, there will be some information lost.
- Tracking programs: Programs that install themselves through internet pages and track your internet usage to transmit data back to their hosts usually have registry keys associated with them. Some have multiple keys, under multiple program names or in multiple folders, within the registry. In order to fully eradicate the program from your computer, you must delete all of its associated registry keys. This can be very tricky.
- Uninstalled programs: If you have recently uninstalled programs without going through the official uninstall program (or if the program did not have an uninstall feature and you deleted it) there may be some keys remaining in the registry. Without removing these keys you may encounter errors as Windows will try to operate based upon the keys. In order to fully uninstall the program, you will need to remove the associated keys.