Problems - Can't get windows to load

I just put my rig together but when i turn it on, windows starts to boot but then there a blue screen that flashes for a 10th of sec (i assume it's a BSOD) and the thing restarts, i cant get past the POST. what could be causing this?

im not a regular member here so here's my rig:

amd 3700+ SD
dfi nf4 ultra-d
256mb of value ram (my gig of UTT arrives next week)
xfx 7800gtx
OCZ modstream 520W
the HD that im trying to boot from is the one that im using right now in my other comp it's a 40gb IDE

I will say as a few others will propably say.
Is it a clean install of windows you are booting? If not format and reinstall.
Is your left hand on your right side? If so go directly to bed
Ok, more of a serious note, help is an answer away.
I won't guess on the PSU yet, but it can be a factor.

//Christian

it's an old install but im not gonna format it, it's not the HD im gonna use in it, the SATA for it is still clean but i dont have an XP cd

did you just take out the HD and stick it in another computer? if so this could be why, you can't just switch harddrives between computers that have diff. mobo's and hardware