My other computer keeps on restarting

Hello,

I have had problems with my computer from the star my cousin buitl it for me last year in Decmber at first it didnt work then it didnt for a few months till around july now iv waited a long time for him to fix it but he wont. I'v tried most thing i can think of at first i thought it was software so i tried to repair it but when i started to repair it it froze at looking for older viersons of windows, (btw i have windows Xp home edition) so i figgered it was my hardrive but when i went to my friends house and tried using his hardware it didnt work (i have a 2006 serial ata not realy sure what version and so does he ther not exactly the same but i thought it might work) so we switched and tried it his in mine and mine in his but it never workerd no matter what we tried and im sure its not my motherbored beacuse it is new I got it around January 2008 and it was compatible. So now i realy dont know what the problem is if you guys could help that would be great.

p.s. i dont think its an overheating problem beacuse i have 5 working fan 1 for my cpu one for me harddrive and 1 for my power supply and 2 on the ouside pointing in for the rest of the stuff.