From EXPERIENCE...(unfortunately)
a)Poor memory (No1 cause.....*usually*)
b) oc'ed mem (no good for install..fine after)
c)weak psu (cant deliver enough STABLE power)...see below
d) Too may drains on power (CD drives/DVD drives/usb devices/pci , sound cards etc..)
e) Too many IRQ calls at one time (as above)
f) New AGP cards with older version of XP (ie original 2001 XP OS with latest gfx card = BIG no no) its three years out of date (and see point 'c').
Remove/unplug all unrequired stuff for install ('cept modem (?...hah! as if) and gfx card/cd drive) ensure 'safe' memory and fsb timings and then try. After install, install other comps and play with timings.