Attempt to connect to Oracle returns an ORA-03124 no interface driver attached.
If you were able to connect and have recently installed another application, the Path or the Registry may have gotten corrupted.
Check the Registry by going to Start|Run and type in regedit. When the folder appears in the left pane, go to HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE\ALL_HOMES and verify how may homes the counter shows. Make sure your last_home entry points to the version of Oracle you are attempting to connect to.
You can edit the home value by double clicking on the last home in the right pane and entering the correct value in the Value_Data box.
To change the path entry go to the command prompt and type in path. The results should have the directory were your oracle client is installed.
To add or change the position of the Oracle entry in win95 or 98 type edit autoexec.bat from the command prompt. Add the entry into the document after the references to windows and save the file. reboot and attempt to reconnect.
To add or change the position of the oracle entry in NT go to the control panel|system|environment|path and make sure that Oracle is the first entry. Test that the change has been made by opening a new command prompt screen and typing path. Verify that the change has been made. Re attempt your connection to Oracle.