On the failing server open Server Manager, and under Roles select Application server. You will most probably find quite a lot of DCOM errors. Open one of those errors to see for which user and for which service the authentication is failing. In the open window copy the GUID of the failing service. It will help you identify the service.

Next open registry editor and select the root node. In Edit menu select Find, paste the GUID and click Find next.

You can find the service name in one of the registry’s values (as seen on the screenshot below, in my case it was IIS WAMREG).

Leave the registry editor open (you might still need it).

Next in Start menu under Administrative tools click Component Services. Expand the Computers node and My Computer. Select DCOM Confing. Now locate the service you identified in your registry, right click it and select Properties.

In the open dialog select Security tab and in the Launch and activation permissions group click the Edit button.

If you have this entire tab grayed out, you need to do the following (outlined in the gray, otherwise skip the gray part below):

In registry editor (which I hope you still have open), right click the node you found and select Permissions Click Advanced and select the Owner tab. Change the owner to the account you’re logged on or to the administrators group (which I presume your account is in) Click Apply and in the permissions dialog also give same user/group full control Now you should close and reopen Component services and find the DCOM service.

In the permissions dialog for the service grant the failing user (or group the account is in) Local Launch and Local Activation permissions. And in case the event log says it is failing remote launch, grant the user remote launch and service also.

Confirm all the changes and close registry editor and Component services window.

Even though at some point when I’d managed to logged in and see my desktop it was impossible to start anything service-based (IIS, SharePoint, Internet Explorer,…)

What did the trick for me? Couple of things, not sure which is THE fix, and I’m adding couple more that I’ve found searching the Web.

1. Reboot into safe mode after applying Windows Updates. This avoids any driver/service conflict issue. After successfully rebooted in safe mode, restart to normal boot.

2. Delete temporary files (to find them, open a "Run" window, type %temp% and press Enter.). This removes the chance of having some legacy corrupt update process file.

3. Check disk space on your system drive – no need for explanation there. Patches can be quite space consuming.

4. If you have IIS (if you have SharePoint, then you do), make a small registry fix to enforce service startup dependancy. Detailed instructions here: http://support.microsoft.com/kb/2004121.

Hopefully one of those "fixes" will work also for you if you’re facing the same problem.

1. You’ve created a web application and you’re sure that credentials are right, but you’re unable to login to the site collection in that web application.

2. Search index log returns strange errors about not finding the web application or access denied.

Then the solution is in the following article:

http://support.microsoft.com/kb/896861 

This did the trick for me:

1. Start regedit
2. Open key HKEY_LOCAL_MACHINESYSTEMCurrentControlSetControlLsaMSV1_0
3. Create a new multi-string value called "BackConnectionHostNames"
4. Type web application’s host name(s) in the Value Data box.

Save and close regedit. Then using Services under Administrative tools restart the IIS Admin service