Windows boot up problem

Recently, I have been unable to bootup my pc.

The system I have is quite old although it is still effective.

OS - Windows XP (SP1)
CPU - AMD 1800
RAM - 512MB
H/D - 40GB

I can get through the bios part ok and initially it seems that Windows is loading as the Windows screen appears and the blue bar moves along (I am using SP1 as I cannot install SP2 without my pc crashing).

It's only after this process that the screen goes blank and my monitor displays a message 'No Video Signal'. At this point I cannot do anything but turn the pc off.

I can however, load up Windows under Safe Mode but don't know where to begin looking for the problem.

Any suggestions would be greatly appreciated.