scree blank part2

Hi,
Part2:
I reinstall my OS (MS WINDOWS98) therefore, now problem during watching the movies ..but when I shutdown my system normaly.. after 2 or 3 houre later I again restart my computer there is SCREEN BLANK (monitor power on) and CDROM AND HARDISK POWER conitneously ON ,shutdown again and again problem face same..went to the vender where I purchase the systme ..He put my processer 400 mhs celeron and my DIM (RAM) another 810chipset motherboard. Its ok .. what the hellis the poblem my motherboard.. please help me
Nawaid