wronghead

Hello everyone. This is my first post here, I have been trawling the internet all day for a solution. I have recently reformatted my computer and wiped it, and put on windows vista ultimate edition. The problem I had before and the reason why I wiped It was because I was getting a blue screen of death bios loop on xp, as soon as the computer got to the windows loading bar it restarted... Anyway, installed Vista Ultimate with no problems atall, for the last 2 weeks or so anyway. But today I seem to have come accross a problem similar to the XP issue. Computer loads but straight after the (hit any key to continue, F6 for bios etc) the computer hangs on a black screen with a white flashing cursor. Now I have done various things that may have triggered this issue today. 1. I updated my java 2. I Installed the newest nvidia drivers for my graphics card (gforce 6800) 3. I disabled windows defender as I am using Opera and was told it is 100% safe to use as it only has 2% market share I downloaded and burned the vista recovery disk. I can get into that OK. It detects no start up issues, however system restore, even back 3 days still dosent fix the problem. any idea what this could be? My PC was a custom build and I can't seem to get into anywhere to access the spec but i suppose I can sort that out if anyone would like to know. Help...much appreciated :)