Its working now.
I'm pretty sure it should be stable at the settings it automatically chose, but its not.
I went into the BIOS and manually changed the external FSB to 266mhz, and this then gave me an option to set the RAM speed, which I changed to Default, turning it to 798mhz again.
I then saved, and Windows boots perfectly.
It seems like the automatic settings of FSB and RAM wasn't booting, making the RAM 816mhz which it didn't like.
Least I know what does it now
The bad news is, I have to now put the damn front board headers back in because I took them out thinking I may have wired them up wrong.