grab your OS cd.
as the computer is starting up hold or tap delete to enter the bios
enter your bios password if you set it
look for the loading of devices for boot startup
CDROM C: D: A:
depending on your bios rotate them so the CDROM is the first.
restart your pc and if your using win98se as an example you can boot to dos with cdrom support.
once u have done this change directories to the windows folder on your c drive.
cd d:
cd win98
from here u can run a scan disk on your pc
scandisk c:
let it run and it may find chunky clusters which you can repair.
after this change the bios back to your hard drive and reboot with the cdrom out.
If it still doesnt let you into the normal booting mode then run msconfig from Start/Run and check the general tab window.
if selected startup isnt activated then toggle normal startup and reboot after apply ok
Once your back into the OS save your data to a recordable device.
reply back if you get stuck or have another OS and it doesnt work.