My friend, this could be a dozen problems at this point. When you took it in to the service dealer, what exactly did he tell you?
How long have you had this computer?
Have you cleaned the computer lately? When I mean clean, I mean reseat all the circuit cards, look for corrosion, clean all the fans, check for loose wiring.
Oh, disclaimer, "UNPLUG COMPUTER BEFORE OPENING"!
I am like you, I want my computer to work and now! Don't beat up your computer we will find out whats wrong soon.
This is something I want you to do. Shutdown your internet connection and your computer. Remove the Ram and place in another socket. You should have two at least or if you have two 256mb sticks remove both. Now place one in the second socket.
Now restart your computer and pay attention. You should here the fans start up and the hard drives start. If you hear both then all is good with most. If it shuts down at this point or stops. Then remove the Ram and place the other stick in and repeat the above.
What we are doing is checking your Ram and connections. If your computer starts up to Windows, immediately go to your system maintenance program and run a quick fix.
System maintenance programs can be Norton, System Mech, McAfee, and such. This is all done offline!
If you can run maintenance, then run a anti-virus program and if that works shut down your computer.
Now I want you to check for heat. Don't touch but run your hand by the ram and CPU. If you feel a real hot from either then this could mean your power supply is bad.
Let us know,
Your Hardware Guide.