If a Windows computer does not start and  you have checked that there was no hardware fault, then there might be something wrong with the software or, in other words, with the operating system. Most of the faults, related to the software, emerge because of malware, viruses, not compatible drivers, old operating system, etc. Viruses are so widely spread and there are so many different types of them that it is the most frequent problem. Some of them also manage to disable antivirus software.
There are some tools to solve it. Firstly, there is the Avira AntiVir Rescue System. This is a bootable CD which functions to scan for viruses. You just have to burn it to an empty CD disk, boot the computer using it and follow the instructions, appearing on screen.
For Windows XP it would be a good idea to test, whether operating system is still healthy. To do that, you should go to Start -> Run -> type “SFC /scan now” and push the ENTER button.  There the Windows XP installation disk will be required.
Another way to fix Windows is to maintain Windows recovery. If you booted the computer, using Windows installation disk, it would detect that the Windows OS is already installed on the computer and would inquire, whether you wanted to perform the Windows recovery. Then you should click “yes” and it would try to recover the system.
The viruses mostly tend to spread by the Internet (e-mail, files, Skype) and by portable media (USB drives, external USB hard disks). To avoid viruses drives, which are going to be connected to a computer, simply must be checked.
Best antivirus programmes are: Avira, Norton, AVG, etc. However, antivirus programmes are, in fact, showering the system, because they check every file, used by operating system. All antivirus programmes do slow down the computer’s work, but some programmes do it to larger, some to a smaller extent.
There are plenty of tools to clean the Windows system. One of them would be JV16 Power Tools. We recommend this one, because some of the programmes included there are in development from 2004. We think the trying it out will speak for itself. (http://www.macecraft.com/jv16powertools2009-info/)