What are appdef.dat Error Messages?

Dynamic Link Library appdef.dat Errors

appdef.dat is considered a type of Dynamic Link Library (DLL) file. Dynamic Link Library files, like appdef.dat, are essentially a "guide book" that stores information and instructions for executable (EXE) files - like MpSigStub.exe - to follow. These files were created so that multiple programs (eg. 360 Total Security) could share the same appdef.dat file, saving valuable memory allocation, therefore making your computer run more efficiently.

Unfortunately, what makes DLL files so convenient and efficient, also makes them extremely vulnerable to problems. If something happens to a shared DAT file, either it goes missing or gets corrupted in some way, it can generate a "runtime" error message. Runtime is pretty self-explanatory; it means that these errors are triggered when appdef.dat is attempted to be loaded either when 360 Total Security is starting up, or in some cases already running. Some of the most common appdef.dat errors include:

• Access Violation at address - appdef.dat.
• appdef.dat could not be found.
• Cannot find C:\Program Files (x86)\360\Total Security\ipc\appdef.dat.
• Cannot register appdef.dat.
• Cannot start 360 Total Security. A required component is missing: appdef.dat. Please install 360 Total Security again.
• Failed to load appdef.dat.
• The application has failed to start because appdef.dat was not found.
• The file appdef.dat is missing or corrupt.
• This application failed to start because appdef.dat was not found. Re-installing the application may fix this problem.

Your appdef.dat file could be missing due to accidental deletion, uninstalled as a shared file of another program (shared with 360 Total Security), or deleted by a malware infection. Furthermore, appdef.dat file corruption could be caused from a power outage when loading 360 Total Security, system crash while loading appdef.dat, bad sectors on your storage media (usually your primary hard drive), or quite commonly, a malware infection. Thus, it's critical to make sure your anti-virus is kept up-to-date and scanning regularly.

How to Fix appdef.dat Errors in 3 Steps (Time to complete: ~5-15 minutes)

If you're encountering one of the error messages above, follow these troubleshooting steps to resolve your appdef.dat issue. These troubleshooting steps are listed in the recommended order of execution.

To begin System Restore (Windows XP, Vista, 7, 8, and 10):

1. Hit the Windows Start button
2. When you see the search box, type "System Restore" and press "ENTER".
3. In the search results, find and click System Restore.
4. Please enter the administrator password (if applicable / prompted).
5. Follow the steps in the System Restore Wizard to choose a relevant restore point.
6. Restore your computer to that backup image.

If the Step 1 fails to resolve the appdef.dat error, please proceed to the Step 2 below.

You can uninstall 360 Total Security software by following these instructions (Windows XP, Vista, 7, 8, and 10):

1. Hit the Windows Start button
2. In the search box, type "Uninstall" and press "ENTER".
3. In the search results, find and click "Add or Remove Programs"
4. Find the entry for 360 Total Security 10.6.0.1223 and click "Uninstall"
5. Follow the prompts for uninstallation.

After the software has been fully uninstalled, restart your PC and reinstall 360 Total Security software.

If this Step 2 fails as well, please proceed to the Step 3 below.

360 Total Security 10.6.0.1223

Qihoo 360 Technology

When the first two steps haven't solved your issue, it might be a good idea to run Windows Update. Many appdef.dat error messages that are encountered can be contributed to an outdated Windows Operating System. To run Windows Update, please follow these easy steps:

1. Hit the Windows Start button
2. In the search box, type "Update" and press "ENTER".
3. In the Windows Update dialog box, click "Check for Updates" (or similar button depending on your Windows version)
4. If updates are available for download, click "Install Updates".
5. After the update is completed, restart your PC.

If Windows Update failed to resolve the appdef.dat error message, please proceed to next step. Please note that this final step is recommended for advanced PC users only.

If Those Steps Fail: Download and Replace Your appdef.dat File (Caution: Advanced)

If none of the previous three troubleshooting steps have resolved your issue, you can try a more aggressive approach (Note: Not recommended for amateur PC users) by downloading and replacing your appropriate appdef.dat file version. We maintain a comprehensive database of 100% malware-free appdef.dat files for every applicable version of 360 Total Security. Please follow the steps below to download and properly replace you file:

1. Locate your Windows operating system version in the list of below "Download appdef.dat Files".
2. Click the appropriate "Download Now" button and download your Windows file version.
3. Copy this file to the appropriate 360 Total Security folder location:
   
   Windows 10: C:\Program Files (x86)\360\Total Security\ipc\
   
4. Restart your computer.

If this final step has failed and you're still encountering the error, you're only remaining option is to do a clean installation of Windows 10.

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