yes, what htummala said is right. if you are connecting to some server on another machine over the network, you need to configure the tnsnames.ora file, so that it will connect properly using net services.
what options you chose when u install oracle? do u remember? let us know that. Don't try re-installation now. try to solve that problem without re-installation.
and one more info, there is one tool called oradim, this will start the oracle services if it is not running. try that also.