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Troubleshooting a Corrupted Registry

From our short exploration of Process Monitor, you can appreciate that for every piece of software that is installed or run on and even removed from a PC, the system will write, query, or delete entries in the Windows Registry. It is therefore vital that you maintain the Registry carefully and ensure that it is kept in good working order.

It is not only that Windows depends on the Registry; applications also store settings and query Registry keys on a regular basis to guide their operational behavior. When corruption occurs, both system and application failures can often be traced to a rogue or incorrect Registry item that can result in error messages and sometimes cause a system to crash.

The main causes of Registry corruption on PCs are...

  • Windows Registry corruption
  • Software installation or application failure
  • Hardware or device driver failure

Windows Registry Corruption

The Registry is a complex database that on occasion can become corrupted. This is, however, quite rare, due to the approach that Microsoft has adopted and refined over the last 30 years of developing Windows. Let us explain.

The Registry is implemented and managed by an executive subsystem called the Configuration Manager (CM). The Registry is made up of several independent Hives, which are then brought together for viewing by the Registry Editor. The CM organizes the Hives on disk and ensures that whenever an application or operating system component reads or changes Registry values or keys, the Registry is always in a recoverable state-even if the PC suffers a crash during Registry modification.

This is achieved by caching parts of the Registry, which allows for very fast search, write, and commit operations. The Registry has evolved from being stored in a simple, flat file structure to benefitting from advances in database technology, so that each in-memory operation is now treated as a transaction and recorded in a special dual-logging scheme. This approach ensures that the transaction will be rolled back if the task could not be completed due to a system failure. Only completed transactions are committed to the Registry.

In addition to the micro-level transactional processing, modern versions of Windows offer self-healing capabilities with features and services such as improvements to New Technology File System (NTFS) and automatic disk check and repair tools, so the underlying file system on which the Registry files are stored remains safe from corruption, thus maintaining Registry integrity.

When a PC is booted, at the beginning of the system boot process, prior to the kernel being loaded (and before the user is presented with the login screen) the Windows Boot Loader also runs some code to ensure the Registry is in a reliable state. If there are inconsistencies, the Boot Loader can perform automatic repairs to fix them before proceeding to load the kernel.

If the system encounters an issue where it needs to invoke a self-heal process, the user will see a pop-up dialog box stating that there was a system error and that the error is being repaired. Once this is completed the PC will continue to load normally.

Software Installation or Application Failure

We have already discussed that during its installation and subsequent configuration, application software writes new Registry keys and updates others with the necessary data to allow the software to work and for Windows to interoperate with the application. When you use a tool such as Process Monitor, you can witness many hundreds of thousands of key entries and configuration changes to the Registry during a typical installation. Even if you deduct from this number an estimate of the Registry changes that occur normally, you will still see many thousands of Registry interactions. Despite advances in how Registry keys are created, modified, and deleted so that no transaction can be left "orphaned" (i.e., partially complete), this is not the case on older operating systems, on which the "blue screen of death" was often caused by the system not being able to read the expected Registry key due to a write error or power outage corrupting part of the Registry.

On all versions of Windows, the Registry is very susceptible to rogue keys being written during software installations; very often numerous keys are simply left in place even after the software has been uninstalled. Much of the time this is due to an element of laziness by the software author and that during processing, the Registry does not check the robustness of the transactions the software installer has provided.

Software vendors often use the Registry to store and maintain licensing information for the software, which is often locked to the specific user account. In some cases the practice of leaving behind licensing status information in the Registry can be a valid strategy-for example, most trial versions of software will not allow the user to uninstall and reinstall the same software on the same PC in order to gain an additional trial period.

The prevalence of malware infecting a PC during software installations is entirely preventable, though this is very much dependent upon the usage of the PC, the user account credentials, and whether UAC is in operation.

In summary, over any period of time, whether this is one year or several years, the Registry on your PC will often accumulate a great deal of unwanted, often obsolete information. Unless malware causes other, more serious problems first, the combined effect of this excess clutter will be a bloated Registry, which can lead to degradation of system performance, increased frequency of application errors, and more system freezes and crashes.

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