XP Freezing during startup

Hello All, I know it is an old problem with lots of possible answers but this one has gotme stumped! I have re-installed XP on this computer over and over and it still freezes during startup (gets to the windows splash screen then the progress bar freezes).
I have now installed only Windows XP MCE, drivers and latest windows updates but it has started to freeze again!
I can only assume this must be hardware related? Memtest86 came back with no errors. Chkdsk says all is fine?
It has an Asus A8N-E motherboard with Nvidia Nforce4 Ultra chipset. 300GB SATA HDD. AMD Athlon64 CPU.
I am out of ideas. Please help! Graham.