I am using oracle 8i on windows 2000 adv srv and getting few errors in system event log repeatedly

1. Following error appears in system event log very repeatedly
"Access denied attempting to launch a DCOM Server using DefaultLaunchPermission. The server is ....
The user in unavailable/unavailable, SID=unavailable."

2. Following error appears once or twice in a day but oracle instance working normally. There is only one instance running in database.
"Instance ORADB has been terminated."

Database performance almost dropped to 25%.

Any help in this regard will be highly appericated.

Thanks
Dreamon