boot screen/os problem

when I turn on the computer, the boot screens from start to the windows xp log in have random characters and lines that block out what usually shows up. the windows xp loading screen is discolored and when i get to the log in screen, it freezes. i also have colored vertical lines on that screen and for the log in and gui even when i run windows xp in safe mode, which is the only mode that works. what could be the cause of this? i have been told it could be a virus attacking the OS. but that doesn’t explain why i can’t format or reinstall windows.