TFS 2008 SP1 resets service accounts
I installed the TFS 2008 SP1 on a site that was using custom accounts for the identities that run the application pools for the WSS instance and Report Services.
These user accounts got reset back to Network Service when the service pack was installed; I had not see this occur on any site I had upgraded previously. This meant you could not start WSS or Reporting Services.
Manually resetting them back to their old correct accounts fixed the problem.
Read the complete post at http://blogs.blackmarble.co.uk/blogs/rfennell/archive/2008/10/16/tfs-2008-sp1-resets-service-accounts.aspx