When you use SQL Developer or Toad, you're able to connect to your Oracle server, but when you try to run a Crystal Report or use TNSPING against your oracle server and you get the following error:

ora-12154: TNS:could not resolve service name

It's possible that your application can not file the tnsnames.ora file.  Make sure that this file exists in the following directory: C:\Ora920\network\admin (NOTE: This path could be different on your machine)

Check your Windows environment variables paths and see if C:\Ora920\network\admin is part of the path.  To check your environment variables on Windows XP, follow the following steps:

1. Click Start > Control Panel > System
2. Click on the Advance tab > Environment Variables
3. Under the System Variables section, look for and select the variable named: Path
4. Click on the Edit button and look for the tnsnames.ora file path
5. If it doesn't exist, add it in and reboot your machine

Hopefully this will solve the ora-12154: TNS:could not resolve service name problem