ORA-12514: TNS:listener does not currently know of service requested

Hello,

I applied the patch from Oracle 10.2.0.1 to 10.2.0.3
(Machine: Windows XP, SP 2)

tnsping works fine.

However, wenn connecting with sqplplus or TOAD, I get the following:

ORA-12514: TNS:listener does not currently know of service requested
in connect descriptor

I get this on the server itself, and also on all client PCs.

Any help is highly appreciated!

Thank you
Georg Scholz