Windows Keeps crashing!! :-(

[DJMEE]

Hi Guys

i recently upgraded my computer:
AMD Quad core 9500
ASUS M2N32-SLI deluxe M?B
500 gig samsung hard drive
2gb DDR2 800 memory (not sure what brand)
650w PSU
i also purchased a 2nd hand XFX 8600GTS and a BFG 8600GTS to use in SLi

i built it putting into my old case and used my old floppy and dvd roms then installed windows XP 64-bit.

i bought the XP 64 for my old system whch had the AMD 64 cpu put t always crashed so in the end just renstalled XP home whch never had any problems.

so on my new system XP 64 ept crashing nothing new so reverted back to XP home again - but now that is crashng too, a few times a day!! sometimes it goes to a blue screen other times the comp just re boots, but in all cases when XP re boots, i get a message saying windows has recovered from a serious error.

to try and see if it may have been the graphcs cards, i tried each one on its own, so i installed my old GE7300 which i've never had a problem with and that hasn't cured the problem so i'm assuming its not the graphcs cards

any ideas on what it could be??
cheers
Q
ps i have updated all dirvers, the only thing i haven't updated is the bios for my motherboard which ASUS only just brought out, with a warning only update as a last resort!!