"Signing in to .NET Messenger Service failed because the service is temporarily unavailable. Please try again later."
This error is usually caused by Windows being unable to find a .dll file that is needed by MSN Messenger for security.
To resolve it, you can
download this file and run it. It will register all the required .dll files which means Windows can find them.
If you prefer, you can also type the following commands yourself to register the .dll files
First open a command prompt by clicking start, then run. In the "run" screen, type: cmd and press enter. You get a command prompt window now. Now, type the following commands and press enter after each command:
regsvr32 softpub.dll, regsvr32 Wintrust.dll, regsvr32 Initpki.dll, regsvr32 Dssenh.dll, regsvr32 Rsaenh.dll, regsvr32 Gpkcsp.dll, regsvr32 Sccbase.dll, regsvr32 Slbcsp.dll, regsvr32 Mssip32.dll, regsvr32 Cryptdlg.dll.
You can now close the command prompt screen and Messenger should work correctly again.