Your problem has nothing to do with 8.1.5 on 2000 , I have used this version of Oracle on Windows 2000 for some time before switching to the full powered 8.1.6 , frankly I faced some weired symptoms and thus is because 8.1.5 is not certified as compatible with win 2000 , if you can move to 8.1.6 or 8.1.7.

Concerning your database :
- check at your tnsnames.ora file and check that it is not corrupted.
- Try to make a new entry for the database in the tnsnames file using Net8 Easyconfig utility ( from Oracle Menu group )
- Check at the listener , see that if it starts normally or not from NT services panel , if not check at litener.ora file and see if it has an entry for the database , if it fails to get up try to activate it from dos prompt using ( LSNRCTL start listener_name )
- If you can connect nomount no more this may indicate that you have a damaged control files , try to create new ones , if you can move to mount then you may have a lost data file.
- The type of error you mentioned ORA-12560 , is a protocol adapter problem , try turn on tracing and see what it will yield , generally your problem is a Net 8 one .
- When you shutdown the server after a hang ( it happens very much on NT ) the most likely to happen that you have got an operating system failure or Oracle systems files failure but not a database one ( or at least that what happened to me )
- After activating the database service start the database from the server manager too and see what happens.
- You may have created your connection string using 8i new convention thus may lead that you have to use connect string as follows : connect internal/[email protected]

Actually possibilities are so many , I need to get more information to be more helpful.

Thank You