Vista refuses to boot, but will boot in Safe mode

I run a dual core AMD with 4GB Ram. System has a Vista score of 4.
It has been running faultlessly for 2 months. Vista Business
On the weekend it started to refuse to boot Vista. PC boots, does all its
checks, then black screen and nothing!!
Booted in safe mode. It runs perfectly! Ran registry checks, antispyware,
virus checks. No problem.
But it still refuses to boot normally.
Any ideas?? I am stumped!