i think i lost hard drive MBR

Hello,
Last night when i restarted my computer, all of sudden it doesn't dected my windows anymore, when i check from computer bios it shows my hard drive has been detected it SATA 250gb western digital, it doesn't start my windows xp, i thought i lost all of my partition but when i connected another 160gb SATA as primary and 250gb WD as slave, from my computer i can see all of my drives and all of my data is still there and even windows files too, one of my friend said that i might lost MBR , don't know what is it and how to install and not sure if i really lost MBR. please help me, thanks for reading.

also sata 250bg WD now doesn't even boot from windows XP CD. when i try to boot it, it stucks and i have to manaully restart it.