You could have a faulty hardware device like sound, video or cd rom or usb device.
You could have the recycle virus that destroys the system volume information. (my son had this). You can get this virus from usb memory sticks or from web sites that say you need the latest flash update when you already have the latest flash update installed.
if you got this virus you have to reformat and reinstall windows.
try this
Safe Mode Bootup
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Try this, its easy and will not harm anything
Slowly tap the F8 key while the computer is powering up
Just after the screen goes blank, the F8 key will point the computer to a menu where safe mode can be selected.
Note if you mistakenly tap F8 key again, it will continue through to normal boot
Scroll and select Last known good configuration.
This is stronger than doing a system restore in normal mode.
Wait till the boot has finished before clicking anything.
If you able to get in , to see if you got a virus do this.
XP- Right click My Computer Icon >manage >storage >disk managenet. In the RHS you should see you C drive. If C drive is missing then you need to re- image or reformat .