So the time has come... you installed TFS and have the basic installation running and then, one day you realize you need to install Team Build separately from the baseline TFS install.  So you go back to the install CD and open the build directory and do the Team Build install.  No worries until you get to that screen that asks you for the account to use.  Of course the documentation says - use the TFSServices account, but it's been months since you installed that and you can't possibly remember the complex never to be used again password you used.

So think about it and figure - look I'll change the password on the TFS Services account and just update the associated services.  So you do that, you install Team Build and everything is looking good.  No worries, you even install Visual Studio so you have a compiler available for Team Build to actually use for builds.  Then you install SQL Server SP1, and restart the computer.

The computer comes back on line and - TFS is dead.  The web sites all say service unavailable - and by all I mean both your WSS project sites and the WSS administration site.  Your connection via Team Explorer both remote and local is also unavailable.  Oh crap now what!

Well when I hit this point I then went looking in the event log.  The application log wasn't too helpful - it was able to tell me that the problem was related to TFSService connecting to SQL Server, but that was about it.  After a few permission enhancements and protocols being turned on still nothing.  Then I jumped over to the System log.  Here we had something useful - the error message on the event: "A failure was encountered while launching the process serving application pool 'TFSWSS'. The application pool has been disabled."  ahh now this points me to something useful.  Off I go to the IIS Manager into my app pools and there they are the two app pools that use TFSServices as their runtime context (TFSWSS and TFS AppPool).  Update the password, restart the app pools and all is good with the world again.  Note that the 'TFS Admin' app pool doesn't use the TFSServices account.  At any rate in case you run into a similar scenario remember - don't panic - dig the event logs the clues are probably found there.