i have a custom built computer running windows xp pro, im not sure why but one day it started to say that it is running in fail safe mode and asks me to press f1 to continue after that it goes to a screen asking how i want windows to start i normaly hit "last known good config" but oher choices are normal safe mode ext..
after that it put th windows loading screen up then reboots the computer to do the same thing all over again. at first i thought it may be the psu becuse i had a psu that was not enough power for the computer an i thought maybe it was going out. so i replaced it with a 750 watt psu an still the same thing. what should i try to do?

Boot your XP CD and press any key to boot from CD when prompted then press the first R for the Recovery Console.

Choose the system to repair (most likely 1) then enter the password. If no password was set for the Main Admin account just hit Enter.

Then at the DOS prompt type: FIXBOOT and hit Enter

Then type EXIT and hit Enter and the computer will reboot.

This time DO NOT press any key to boot from CD when prompted. Just let it run and see if XP will start.

I have this problem and I noticed another HDD with Linux on it works fine. Which means is driver/software related, most likely.

In many cases its a corruption that FIXBOOT will correct by writing a new partition boot sector to the system partition. See if that works.

ok, i tried the FIXBOOT thing and nothing changed i forgot to mention my HDDs are 250 gig x2 on RAID 0

Other options from the Recovery Console:

FIXMBR

and or:

BOOTCFG

More detail:

http://support.microsoft.com/kb/314058

If no luck then a Repair Install might be your most expedient option.