How do I fix a "Self registration of the DLL c:\winnt\system32\comcat.dll failed : 0d" problem?

When upgrading to a new service pack (specifically, to SP4), some users may receive and error such as:

[color red]Self registration of the DLL c:\winnt\system32\comcat.dll failed : 0d

Processing file <drive>:\Eng\server\setup\I386\event.ins at or near line 22
The specified procedure could not be found
Microsoft Window NT
ID No. 0xc002007F[/color]

This occurs when the comcat.dll version is incompatible with the ole32.dll version. The solution is not the obvious, to upgrade the ole32.dll, but to "downgrade" the comcat.dll.

To see if a version conflict is the culprit, run the following from a DOS prompt:

Windows 9x:

[color blue]cd <path to windows\system>
regsvr32 comcat.dll[/color]

Windows NT:

[color blue]cd <path to winnt\system32>
regsvr32 comcat.dll[/color]

If there is a version conflict, you will receive an error such as:

[color red]Comcat.dll was loaded, but the DllRegisterServer was entry point not found. DllRegisterServer may not be exported, or a corrupt version may be in memory. Consider using Pview to detect and remove it.[/color]

Here's how to fix it by installing and registering comcat version 4.71:

1. Download http://www.vvm.com/~blawson/comcat.dll and save it to a directory on your system other than the <windows\system> or <winnt\system32> directory.
2. From a DOS prompt:
[color blue]cd <path to windows\system> or cd <path to winnt\system32>
regsvr32.exe /u comcat.dll[/color]
3. You should receive a message that the unregistration was successful.
4. Rename the current comcat.dll in the system directory to comcat.dl_
5. Move the file you downloaded to the system directory.
6. From a DOS prompt:[color blue]
cd <path to windows\system> or cd <path to winnt\system32>
regsvr32.exe comcat.dll[/color]

Try installing the service pack again. In most cases, this resolves the problem.