HANG UP

I am helping a friend upgrade his PC. I insalled a new MB, HD, RAM, DVD and DVD _RW, and Processor. He is running XP Pro. Everything seems to run fine and after a while the system just stops responding. I have downloaded every update and service pack from Microsoft and have run Norton Utilities. Same problem. He has a new ASUS P4PE-X board with 3.06 GHz P4 and 512 Mb RAM. The DVD is a Sony and the DVD -RW is TDK. the HD is a Western Digital. Also, He has a new case fan and a second expansion slot fan. I am about at my last straw, can anyone help?