Please tell us the brand and model. If not known, look for the model number on the center area of the mother board, (if you know how to remove the cover).
If you know how to get at it, tell us the brand of the hard drive.
This sounds as if you have a failed hard drive... but could be a power supply... (some devices work on one part of the power supply, while other parts are defective).
Can you tell if the video graphics socket is part of the motherboard, or is on a pci slot near the modem?
The problem is, it is difficult to guess at the problem: It can be power supply, hard drive, cpu fan, cpu thermal paste, video graphics card or socket or driver, memory module which has failed.
We need as much information as you can provide, so as to make the best guesses at a solution.
Do you have the original Windows disk, or computer recovery or restore disk set?
Do you have valuable data installed?