How to fix windwos error c000021a

**Walter89** · 25-05-2009

I am getting a error message c000021a with blue screen on my system. The blue screen is come only after a windows updated was downloaded and installed. The system was working great before that. The update was not properly installed due some windows installer issue. I believe that because of that, the error occurred. It had happened two or three times. What are the steps to fix this error. The OS is windows xp.

**Raine** · 25-05-2009

The basic cause is windows update. Because some windows update are not compatible with the system. There can be also a RAM problem with your system. Try to replace a RAM and then work on your system. If still the blue screen or the error appear then there is definitely the update issue. Try to restore your computer. You can restore by two ways first. Go to Start Menu > Programs > Accessories > System Tools > System Restore. And second way navigate to path C:\Documents and Settings\All Users\Start Menu\Programs\Accessories\SystemTools\system restore. Launch the application and restore your pc to an earlier time. By this windows will remove the update file and restore your system back as it was.

**Eric B** · 25-05-2009

You are correct the update is creating a problem in your system. Try to uninstall it form the Control Panel > Add/Remove Programs. Or either try some of the below steps to resolve your issue :

First insert your original Windows XP CD in the CD/DVD media. Then go to Start Menu > Run > In the dialog box type sfc /scannow. It will not work without the CD. Hold down for minute let the scan run.
Now if still you don't get any result then you will need to delete the file instcat.dll from the registry.\
Go to Start Menu > Run > Type regedit
In the Registry editor navigate to the following : HKEY_LOCAL_MACHINE \ SOFTWARE \ MICROSOFT \ WINDOWS NT \ CurrentVersion \ Winlogon \ Notify \ instcat

Deletd the instcat file and restart your system

**chroma** · 25-05-2009

Download the software PC Optimizer Pro. Run this tool on your system. This tool will optimize your system performance. Otherwise the best option is to repair your windows xp version. It will delete all your data and software you had installed. So it is much better that you have a back up of it. To repair the OS follow the below instructions :