Bootup failure can be due to a variety of issues to include application faults, hardware failures, or malware. Startup failures that occur before
the OS loader (Ntldr) starts could indicate missing or deleted files, or damage to the hard disk master boot record (MBR), partition table, or boot sector. If a problem occurs during
startup, the system might have incompatible software or drivers, incompatible or improperly configured hardware, or corrupted system files.
If Windows XP (or 2000) refuses to start in normal mode, press F8 repeatedly right after you turn on your PC but before the Windows log-on appears and select "Last Known Good Configuration
" or System Restore from a command prompt in Safe Mode
to return to a previous state before your problems began.
If, that does not work then see:
"What to Do When XP or 2000 Won't Boot
"10 things you can do when Windows XP won't boot
"Langa Letter: XP's No-Reformat, Nondestructive Total-Rebuild Option
"How to recover from a corrupted registry that prevents Windows XP from starting