I have been working with a customer who ran into error code 23 while trying to download Media Center guide information after installing Update Rollup 2 for Media Center 2005.  Using the steps listed in this previous blog post, we tried to repair the BITS service using the bitsadmin.exe tool that is part of the Windows XP SP2 support tools package.  Running that tool gave an error like the following:

Failed to instantiate BITS Manager interface... - 0x80080005
Server execution failed

After talking to the BITS team, I had the customer try the following workaround and it solved the issue on this machine:

1. Check to see if the registry sub-key named HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\BackupRestore\FilesNotToBackup exists on the system
2. If not, click on the Start menu, choose Run and type cmd
3. Run the command reg add HKLM\SYSTEM\CurrentControlSet\Control\BackupRestore\FilesNotToBackup /f in the cmd prompt window
4. After adding this registry value, try to start the BITS service by running net start bits

I am not sure how that registry value got deleted on this machine, but adding it back allowed Media Center guide download and other BITS-related download functionality such as Windows Update to start working again.