Can't boot a healthy WinXP

Here it is: When turned on, it shows the standard black pre-windows screen where he recognizes the hardware configuration then the screen sort of switches off for a second as usual and then normally the WinXp black screen with the progress bar would come on. But it doesn't. The screen stays blank and nothing happens. At first it happened once a week, now it happens all the time. Only once in 6-7 attempts it manages to go through and boot windows. Today it even restarted once by itself with no active programs.

I'm 100% sure the windows installation is ok. I know because I first thought windows was bad so I reformatted and did a perfect reinstall and just installed the drivers and a few programs that I've already used before without problems. Didn't even connect to the internet. This reboot problem didn't disappear. The hardware configuration didn't change for a long time and the basics are:
Athlon xp 2000+, 512 MB ddram, seagate HD 120GB, nvidia 64 mb..
Heelp!