system restarts suddenly!!!!

i was previously using intel D102 with ddr2 533mhz ram with dual core 2.8ghz processor..a couple of days ago i bought a new intel 946GZIS and core2duo 1.8ghz E4300,ddr2 512mb 667mhz ram and new SMPS, 160gb SATA, LITE-On 20x DVD writer and an old cd rom...after i installed windows xp, sometimes when i open some applicatons and work or if i see some movies suddenly my systm gets restarted...after my windows starts again i get a msg tat "windows has recovered frm a serious error" n bla bla bla..i tried changing the ram,but still i had the problem.. then i tried changin the smps still i faced the same problem.. then i formatted my system and reinstalled windows xp yet again i face the same problem.. pls help me wat cud b the reason.. i chked the cpu cooler also and i even chked the temperature... its optimum..