new computer BSOD

well, my machine is still suffering from occasional BSODs. at this point, it seems to only ever crop up while the machine is booting, just after it starts trying to load Windows. but it doesn't happen -every- time; maybe about 1 in 4. i've swapped position of my RAM (thus reseating them as well) and i've slightly backdated my vid card drivers. however, the screen flashes by so quickly that i can't capture any specifics at all on the error.
i have -not- yet run Windows Update: i didn't buy a new copy of XP, and i'm loathe to counterfeit my old machine's OS w/out the new machine being stable..

any ideas? or any advice on who / where i should ask?