the problem is that my system for no reason will just reboot its self

system spec

epox 4pda2 motherboard
P4 northwood 3ghz with ht
1gb twin mos dual channel ram
seagate 80gb
seagate 200gb
phillips cd rw
pioneer dvd
thermaltake butterfly power supply
thermaltake armor case
asetek watercooling
geforce 4 mx 440 graphics card
zalman fan speed controoler
windows xp

as above it will just reboot it self with no warning and i have no idea why, temps are fine and voltages seem to be stable ( although i do not no what they are suposed to be ).

please help as its getting realy annoying
thanx