Oracle has put its directories in front of the others in the PATH environment variable and now the old Java comes sooner in the path. You need to move the path to the version of Java you want to use to the front of the list in the PATH variable.
I suppose you're on Windows. You can edit the path using Control Panel -> System -> Advanced -> Environment variables (the exact way depends on your version of Windows, please look the proper way for your version on the net if needed). Do not remove any entries from there, just reorder the paths (they are separated by semicolon and the editing can be quite cumbersome, as the value is much longer than the edit field.)
subject: java runtime environment is changing when i install Oracle 10g