NTDLR file missing

I currently put together a Pentinum 4 2.6 gig 800MHZ on a Soltek SL-86SPE-L Socket 478 800MHZ mainboard with 8x AGP and SATA support. In the board i have two 256MB DDR 400 Corsair Ram and two 120gigs SATA Seagate HDD with XP Pro running. Everytime in between four or five hours or so when I switched on the computer it simply came up with a message that NTLDR file is missing press CLTR, ALT & DELTE to restart. Then the same thing came up over and over again. So I press Power off and then on again then the XP LOGO finally came up. Actually the file is in the Window's system folder. I checked on the Bios and everything seems to be fine. Now even funnier things happened is the American Trend booting screen booting back and forth for six times before XP Logo show up. Does anyone know what's going on.
 
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