It sounds as though you may have some hardware going out on you. Since the monitor says no signal I would start with the video card. If you have an onboard video card you may want to get a PCI or AGP video card to install in your computer to see if the onboard video is going out. I would advice you to get a PCI card in this case if you don't have an AGP slot and your motherboard already have integrated graphic so should buy a PCI card to check whether the AGP port is gone bad or else try using another video card to check your display.
It could also be a problem with other hardware such as your motherboard, memory or processor. If any one of these are bad it will lot let video be displayed on the computer.

There is also a chance it is software related. I would boot into Safe Mode and let it run for awhile to see if it crashes there too. You can also boot to a Windows CD or other bootable CD and let the computer sit there without going into Windows to rule out a Windows problem. If it stays running fine without being in Windows then you know to start looking into a Windows repair or reinstallation