It could be a hardware problem or virus/malware. Try to boot from a CD and run some diagnostics http://www.tech-recipes.com/rx/469/boot_cd_rom_run_diagnostic_tools/

>
>I have Windows XP computer that won't start.
>
>If I start in safe mode it displays a few files and then hangs.
>
>I've tried repairing the installation and it runs the initial repair copy ok but then when it comes to restarting the system to complete the repair it hangs again.
>
>Any thoughts on what could be wrong ? or things I could try ?
>
>Thanks
>
>Nick