If a computer powers on but does not load Windows, either the installation is corrupt or the hard drive is faulty. Acquire a recovery disk from the Microsoft website, turn on the PC and press Delete to enter the BIOS. Under the Boot tab, select the DVD drive as the first device, insert the disk and select "exit and save changes." Follow the steps to fix the PC.
If the PC still refuses to boot, follow the same steps to re-install Windows from scratch, using the code on the PC to activate the system.
If the PC still doesn't boot, the hard drive needs to be replaced. Remove the hard drive and use it as a reference to obtain a new one.
If the PC doesn't turn on at all, either the power supply or the motherboard is likely to be faulty. Use a power supply tester to determine if the power supply is working. If it is, change the motherboard. Some PC's don't power on if the processor is faulty. If changing the power supply and motherboard doesn't work, replace the processor.
If the PC is beeping when it turns on, there is a hardware problem. Different beep codes indicate different problems. The code tells what is wrong with the computer. Replace the problematic part, and the PC should work.