Loading xp machine keeps rebooting in midle

Hello,
im reloading xp on a mecer xtreme machine and it keeps rebooting in the middle of loading and gives no error masage at all.sotimes reboots when win is checking hardware configurations,sometimes it goes further and gets to a stage were i have to format then restarts while formating.tried removing cards,using diffirent memory modules using diffirent HDD nothing makes a difirance.plz help im about 2 go mad.