I have a Dell Optiplex 760 running Windows 7 Home Premium. It has 2 gigabytes of RAM, Intel® Core™2 Duo Processor E8400, 160 GB hard drive.I recently bought this computer and it has been working perfectly. When I tried to start it this morning, the 3 & 4 lights were on and it would not start. I need help diagnosing the problem. Thanks in advance.