Gozar,
Start with the basics. Computers that have been off the power grid for a long time need a warm boot because of dead PRAM batteries. Turn on the compter, let it 'warm up' for a few seconds. Now hold down the 'command' key and the 'control' key and then press the 'soft power' key on the upper right corner of the keyboard ( may be top center on some keyboards).
Then post back with the sound that it makes. Chimes, breaking glass, musical chords, what ever. Allow time for it to cycle through its chores. Make note of any error code it might give.
If you do not hear chimes turn on the power. Wait for noises. Make note of which noises you hear, ie. power supply fan comming on. Try turnning power off and right back on immediately.
Also, zapping the PRAM might be called for depending on how responsive the machine is. You may need to do surgery to clean contacts, etc. but start with simple and proceed from there. As for the hard drive? did it work in the past or did someone put it in, fail to get it working so they sold the TAM to you?
Good luck. Post back with details.
Jim